add crc
add pit
[pit] disable timer interrupt for null callbacks
[crc] add xor read complement. simpler CRC32 init
Merge pull request #80 from tralamazza/crc

add crc
[pit] retab
Merge branch 'pit' of github.com:tralamazza/mchck into tralamazza-pit
Merge branch 'master' of github.com:mchck/mchck

Conflicts:
	toolchain/ld/MK20DX32VLF5.ld
	toolchain/lib/mchck/Makefile.part
	toolchain/lib/mchck/mchck.h