Call pre/post-install in factory installer

This would allow us to run board-specific pre/post install scripts. For
example, if we need to do pre-install check, we can install a
board_factory_pre_install.sh script, and it will be executed at the
beginning of factory install process.

BUG=chrome-os-partner:19771
TEST=None

Change-Id: I911f425f1013da95172c93ba06992ccff6d605f1
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/172411
1 file changed