Added Circuit Playground Express board

boards.txt

bootloaders/circuitplay/circuitplay_m0_samd21g18_sam_ba.bin

drivers/Adafruit_CircuitPlay_Express.inf

drivers/cdc_vista.cat

variants/circuitplay/debug_scripts/variant.gdb

...s/circuitplay/linker_scripts/gcc/flash_with_bootloader.ld

...ircuitplay/linker_scripts/gcc/flash_without_bootloader.ld

variants/circuitplay/openocd_scripts/arduino_zero.cfg

variants/circuitplay/pins_arduino.h

variants/circuitplay/variant.cpp

variants/circuitplay/variant.h