libpayload: Expose options_checksum_valid

options_checksum_valid can be used as a fast test to
identify invalid CMOS data by checking the checksum.

Change-Id: I44635d4c5d389579ad82435907ba8658e1bd44bb
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/586
Reviewed-by: Bernhard Urban <lewurm@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>