add crc
spi: Handle NULL rx buffer

This allows one to ignore recieved data without a throw-away buffer
add pit
[pit] disable timer interrupt for null callbacks
fix typo
[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
[crc] add crc width enum
[crc] refactoring
fix CRC32 init
add CRC32B init (commonly known as CRC32 -_-)
update crc_update to accept a buffer/len
[crc] crc32b example
pushed by mistake sry
[crc] name not necessary
rtc: Add register definitions
MK20DZ10.h: Remove conflicting RTC definitions
rtc: Add some utility functions
Merge pull request #81 from bgamari/rtc

Add real-time clock support
Merge pull request #82 from bgamari/spi

spi: Handle NULL rx buffer