xum1541: Fixed compilation with recent avr-gcc releases.

* Removed -Werror to ignore warnings in the LUFA library.
  Actually, there are a lot of new warnings with avr-gcc 4.7.2 which
  should perhaps be fixed sometime (maybe by updating LUFA?).

* Declared PROGMEM variables as const.
xum1541: Added teensy2 board.

The PJRC Teensy 2.0 is a very small, low-cost board with an ATMEGA32U4
controller. It's available from http://www.pjrc.com/teensy/

No additional components are required - all the IEC lines can be
soldered directly to the board.
xum1541: Added hex file for the teensy2.

compiled with avr-gcc.exe (AVR_8_bit_GNU_Toolchain_3.4.2_939) 4.7.2