Keep ARDUINO_SAMD_ZERO flag for compatilibity

- Instead of removing it in favor of ARDUINO_SAMD_FEATHER_M0 have both
  flags present to increase chances of compatibility with libraries.

Author: obsoleted

Diff for: boards.txt

@@ -32,7 +32,7 @@ adafruit_feather_m0.build.usb_product="Feather M0"
 adafruit_feather_m0.build.usb_manufacturer="Adafruit"
 adafruit_feather_m0.build.board=SAMD_FEATHER_M0
 adafruit_feather_m0.build.core=arduino
-adafruit_feather_m0.build.extra_flags=-D__SAMD21G18A__ {build.usb_flags}
+adafruit_feather_m0.build.extra_flags=-DARDUINO_SAMD_ZERO -D__SAMD21G18A__ {build.usb_flags}
 adafruit_feather_m0.build.ldscript=linker_scripts/gcc/flash_with_bootloader.ld
 adafruit_feather_m0.build.openocdscript=openocd_scripts/arduino_zero.cfg
 adafruit_feather_m0.build.variant=arduino_zero