lib: add crc7.

Used as a checksum on command messages when talking with MMC cards.

Implemented using the unwound bytewise implementation from
https://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks
which is a good mix of size and speed.

The API and naming matches lib/crc7.c in Linux.

Signed-off-by: Michael Hope <mlhx@google.com>
4 files changed