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 50ff9afb)
Signed-off-by: Steve Sakoman <steve@sakoman.com>