Move firmware to a subdirectory

firmware/teensy3/core_cm4_simd.h

firmware/teensy3/core_cmInstr.h

firmware/teensy3/core_id.h

firmware/teensy3/core_pins.h

firmware/teensy3/eeprom.c

firmware/teensy3/elapsedMillis.h

firmware/teensy3/keylayouts.c

firmware/teensy3/keylayouts.h

firmware/teensy3/main.cpp

firmware/teensy3/math_helper.c

firmware/teensy3/math_helper.h

firmware/teensy3/mk20dx128.c

firmware/teensy3/mk20dx128.h

firmware/teensy3/mk20dx128.ld

firmware/teensy3/nonstd.c

firmware/teensy3/pins_arduino.h

firmware/teensy3/pins_teensy.c

firmware/teensy3/serial1.c

firmware/teensy3/serial2.c

firmware/teensy3/serial3.c