gcode_parse.c: process incoming digits earlier.

Idea and patch from ItsDrone, see
http://forums.reprap.org/read.php?146,174364,174550#msg-174550

Thanks, ItsDrone.
Replace #define HOST by #define MOTHERBOARD.

Apparently, HOST conflicts with some AVR headers for the
USB-equipped ATmegas.

Patch by DaveX, thank you Dave. See also:
http://forums.reprap.org/read.php?147,33082,182021#msg-182021
arduino.h: add the AT90USB1286.

Thanks for the patch, DaveX.
usb_serial.c: make variables residing in PROGMEM constant.

Newer avr-gccs complain about that and even without this, it's
the right thing to do.

Patch by DaveX, thanks Dave. See also:
http://forums.reprap.org/read.php?147,33082,182021#msg-182021
Makefile-AVR: fix ...

... well, I've forgotten what this fixed, but it was neccessary
at some point.