libpayload: enforce const correctness for CMOS getter/setter

libpayload: enforce const correctness for CMOS getter/setter

Input only arguments to {get,set}_option*() should be const to catch
programming errors early.

Change-Id: I560001a8e9226dfd156a4e529fcad20549236ebd
Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Reviewed-on: http://review.coreboot.org/652
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>