currently, when M5Stack board is selected (espressif#1242)

M5Stack variant (pins_arduino.h) file is not automatically included,
because boards.txt m5stack-core-esp32.build.variant=esp32
This commit changes m5stack-core-esp32.build.variant from esp32 to m5stack_core_esp32
also rename M5Stack variant directory from m5stack-core-esp32 to m5stack_core_esp32

Author: reaper7

Diff for: boards.txt

@@ -1217,7 +1217,7 @@ m5stack-core-esp32.serial.disableRTS=true
 
 m5stack-core-esp32.build.mcu=esp32
 m5stack-core-esp32.build.core=esp32
-m5stack-core-esp32.build.variant=esp32
+m5stack-core-esp32.build.variant=m5stack_core_esp32
 m5stack-core-esp32.build.board=M5Stack_Core_ESP32
 
 m5stack-core-esp32.build.f_cpu=240000000L