FASTBOOT: Add a hook to allow board files to handle image checks before flashing

Sometimes the image passed by a host via 'fastboot flash' might need
some additional parsing (like to handle files with more than one
image).  Add a hook that board files can implement to do those
extra checks.

Change-Id: I64f053a78ad898dcbef76d53477587535accc3f7
Signed-off-by: Mike J. Chen <mjchen@google.com>
2 files changed