util/cbfstool: Add "expand" command to make CBFS span an fmap region

util/cbfstool: Add "expand" command to make CBFS span an fmap region

vboot images come with multiple regions carrying CBFS file systems. To
expedite hashing (from slow flash memory), the FW_MAIN_* regions are
truncated since they typically have pretty large unused space at the
end that is of no interest.
For test purposes it can be useful to re-engage that space, so add a
command that creates a new empty file entry covering that area (except
for the last 4 bytes for the master header pointer, as usual).

BUG=b:65853903
BRANCH=none
TEST=`cbfstool test.bin expand -r FW_MAIN_A` creates a new empty file of
the expected size on a Chrome OS firmware image.

Change-Id: I160c8529ce4bfcc28685166b6d9035ade4f6f1d1
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/21598
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>