Fix our CMOS checksum algorithm so it matches what /dev/nvram expects

Fix our CMOS checksum algorithm so it matches what /dev/nvram expects

Our cmos checksum is inverted to what the Linux /dev/nvram device expects (and
BIOSes use). This makes it impossible to use /dev/nvram with coreboot. Fix it!

Change-Id: I239f7e3aca05d3691aee16490dd801df2ccaefd1
Signed-off-by: Vadim Bendebury <vbendeb@google.com>
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/279
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>