cros_make_image_bootable: add board-specific hook

This patch adds a board-specific hook so that individual boards can
apply custom changes such as bootloader configuration tweaks.

TEST=build_image succeeds
BUG=chromium:604967

Change-Id: I0d9c00942c4691cafc00c1f03acd32e27cb178d5
Reviewed-on: https://chromium-review.googlesource.com/336724
Commit-Ready: Amey Deshpande <ameyd@google.com>
Tested-by: Amey Deshpande <ameyd@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed