Allow reading a backup without doing an update

If no firmware image is provided, but --backup is requested, still read
the current image and save it to disk.

BUG=chromium-os:21568
TEST=cyapa_fw_update -f -b -o /tmp/backup.bin
  => Should backup current firmware to /tmp/backup.bin

Change-Id: I10c79a2ead42038583a627024aca39a68731892b
1 file changed