externalsrc: Detect code changes in submodules
The srctree_hash was calculated only from main source directory ignoring
changes in submodules.
[YOCTO #13748]
Use submodule--helper to determine all submodules, and calculate hash
from all git tree objects names combined.
Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit
50ff9afb3990bcf60b4fa1f937506cb84028c32d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
