libpayload: Provide interpretation of CMOS data structures

Add new functions that allow using string based key/value access to
CMOS, including support for enums.

Change-Id: Ibe238eff4c5230e5f61004c88221cd34393873aa
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/585
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>