Move firmware to a subdirectory

firmware/teensy3/Stream.h

firmware/teensy3/Tone.cpp

firmware/teensy3/Udp.h

firmware/teensy3/WCharacter.h

firmware/teensy3/WConstants.h

firmware/teensy3/WMath.cpp

firmware/teensy3/WProgram.h

firmware/teensy3/WString.cpp

firmware/teensy3/WString.h

firmware/teensy3/analog.c

firmware/teensy3/arm_common_tables.h

firmware/teensy3/arm_math.h

firmware/teensy3/avr/eeprom.h

firmware/teensy3/avr/interrupt.h

firmware/teensy3/avr/io.h

firmware/teensy3/avr/pgmspace.h

firmware/teensy3/avr_emulation.h

firmware/teensy3/avr_functions.h

firmware/teensy3/binary.h

firmware/teensy3/core_cm4.h