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.
Add support for the AT90USB1286.

Contribution by DaveX. Thank you very much, Dave. See
http://forums.reprap.org/read.php?147,33082,182206#msg-182206
Clean up some unused delay related stuff.
Eliminate _delay_ms().

Lots of duplicate stuff in delay.c/.h ...
Eliminate _delay(), delay() and _delay_us().

Doesn't change much, even the binary size stays the same. Much
cleaner code though, now we have only delay_us() and delay_ms().
delay.h: re-add falsely removed #include.