As we are only writing to the BIOS region, we only need to backup the BIOS
region. The will avoid the error "failed to back up original firmware" if
regions such as the ME can't be read.
This change allows other device types to derive from common flashrom
code in the flashrom-device type.
Change-Id: Ic963fd586e0a73153c54889fce50055753a1bf5c