Remove GCC -Wextra warnings from build process

[earlephilhower]

Description of Change

When building the core with -Wextra a few locations have case:
fall throughs or skipped field initializers.

Add proper comments for B64 cases to avoid GCC warnings

Initialized unused fields in Tone and WiFiGeneric to avoid missing
field initializer warnings.

Tests scenarios

Builds from the IDE with File->Preferences->Compiler Warnings->All show warnings shown below

2022-05-19T17:40:42.2280300Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c: In function 'base64_decode_block_signed':
2022-05-19T17:40:42.2282122Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c:42:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
2022-05-19T17:40:42.2283247Z          *plainchar    = (fragment & 0x03f) << 2;
2022-05-19T17:40:42.2284240Z          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
2022-05-19T17:40:42.2285087Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c:43:7: note: here
2022-05-19T17:40:42.2285435Z        case step_b:
2022-05-19T17:40:42.2285691Z        ^~~~
2022-05-19T17:40:42.2286515Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c:53:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
2022-05-19T17:40:42.2286932Z          *plainchar    = (fragment & 0x00f) << 4;
2022-05-19T17:40:42.2287219Z          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
2022-05-19T17:40:42.2287609Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c:54:7: note: here
2022-05-19T17:40:42.2287909Z        case step_c:
2022-05-19T17:40:42.2288200Z        ^~~~
2022-05-19T17:40:42.2288972Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c:64:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
2022-05-19T17:40:42.2289491Z          *plainchar    = (fragment & 0x003) << 6;
2022-05-19T17:40:42.2289745Z          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
2022-05-19T17:40:42.2290162Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cdecode.c:65:7: note: here
2022-05-19T17:40:42.2290509Z        case step_d:
2022-05-19T17:40:42.2290714Z        ^~~~
2022-05-19T17:40:42.2482744Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cencode.c: In function 'base64_encode_block':
2022-05-19T17:40:42.2484713Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cencode.c:46:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
2022-05-19T17:40:42.2485415Z              result = (fragment & 0x003) << 4;
2022-05-19T17:40:42.2486713Z              ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
2022-05-19T17:40:42.2487696Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cencode.c:47:9: note: here
2022-05-19T17:40:42.2488519Z          case step_B:
2022-05-19T17:40:42.2489175Z          ^~~~
2022-05-19T17:40:42.2492458Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cencode.c:56:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
2022-05-19T17:40:42.2493351Z              result = (fragment & 0x00f) << 2;
2022-05-19T17:40:42.2494227Z              ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
2022-05-19T17:40:42.2496324Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/libb64/cencode.c:57:9: note: here
2022-05-19T17:40:42.2496937Z          case step_C:
2022-05-19T17:40:42.2497261Z          ^~~~
2022-05-19T17:40:44.6354962Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp: In function 'void setToneChannel(uint8_t)':
2022-05-19T17:40:44.6356417Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:99:5: warning: missing initializer for member 'tone_msg_t::pin' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6357120Z      };
2022-05-19T17:40:44.6358732Z      ^
2022-05-19T17:40:44.6364470Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:99:5: warning: missing initializer for member 'tone_msg_t::frequency' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6367914Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:99:5: warning: missing initializer for member 'tone_msg_t::duration' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6372875Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp: In function 'void noTone(uint8_t)':
2022-05-19T17:40:44.6373943Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:110:5: warning: missing initializer for member 'tone_msg_t::frequency' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6375154Z      };
2022-05-19T17:40:44.6375825Z      ^
2022-05-19T17:40:44.6379852Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:110:5: warning: missing initializer for member 'tone_msg_t::duration' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6383291Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:110:5: warning: missing initializer for member 'tone_msg_t::channel' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6388688Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp: In function 'void tone(uint8_t, unsigned int, long unsigned int)':
2022-05-19T17:40:44.6389829Z /home/runner/arduino_ide/hardware/espressif/esp32/cores/esp32/Tone.cpp:128:5: warning: missing initializer for member 'tone_msg_t::channel' [-Wmissing-field-initializers]
2022-05-19T17:40:44.6390677Z      };
2022-05-19T17:40:44.6391420Z      ^
2022-05-19T17:42:00.6768353Z /home/runner/arduino_ide/hardware/espressif/esp32/libraries/WiFi/src/WiFiGeneric.cpp: In static member function 'static bool WiFiGenericClass::setDualAntennaConfig(uint8_t, uint8_t, wifi_rx_ant_t, wifi_tx_ant_t)':
2022-05-19T17:42:00.6769293Z /home/runner/arduino_ide/hardware/espressif/esp32/libraries/WiFi/src/WiFiGeneric.cpp:1333:5: warning: missing initializer for member 'wifi_ant_config_t::rx_ant_default' [-Wmissing-field-initializers]
2022-05-19T17:42:00.6769658Z      };
2022-05-19T17:42:00.6769824Z      ^

After applying this patch they disappear.

[CLAassistant]

All committers have signed the CLA.

[earlephilhower]

This would also allow enabling -Werror in the CI build scripts, but I didn't feel it was appropriate to do so on my own in this PR.

[me-no-dev]

Thanks @earlephilhower :)