firmware: Update image version string when doing --sb_repack.

In many automatic or manual testing flow, people may use "--sb_repack" to
create a modified updater with different firmware. However they don't always
update the TARGET_* and -V content list, which sometimes caused confusion when
trying to identify if new image is correctly replace in updater.

Since most BIOS/EC version strings are using same format, we should try harder
to update the version in --sb_repack.

BUG=none
TEST=emerge-nyan chromeos-firwmare-nyan # Success.
     chromeos-firmwareupdate --sb_extract /tmp/sb
     cp /build/nyan/firmware/image.bin /tmp/sb.bin
     chromeos-firmwareupdate --sb_repack /tmp/sb
     chromeos-firmwareupdate -V # See new firmware strings.

Change-Id: I7716a82f41c942c0d4d3964a717f80936e2892ba
Reviewed-on: https://chromium-review.googlesource.com/201206
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>