avbtool: Keep FEC data around when using --keep_hashtree with erase_footer.

The expected behavior is to keep the FEC data around so fix the code
to do this (instead of only keeping the hashtree around). Also update
the README file to mention FEC codes.

Bug: None
Test: Updated unit test + unit tests pass.
Change-Id: Ia7e8008327ad92d506de96d31847e3ddad24e5c2
3 files changed