Move cmos.default handling to bootblock

Move cmos.default handling to bootblock

The cmos.default code wasn't actually used so far, due to an oversight
when forward-porting this feature from an old branch.

- Extend walkcbfs' use by factoring out the stage handling into C code.
- New sanitize_cmos() function that looks if CMOS data is invalid and
  cmos.default exists and if so overwrites CMOS with cmos.default data.
- Use sanitize_cmos() in both bootblock implementations.
- Drop the need to reboot after writing CMOS: CMOS wasn't used so far,
  so we can go on without a reboot.
- Remove the restriction that cmos.default only works on CAR boards.
- Always build in cmos.default support on boards that
  USE_OPTION_TABLE.

Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Acked-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6436 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1