externalsrc: Fix parsing error with devtool non-git sources

If srcdir is under poky directory (e.g. devtool poky/build/workspace/sources)
and is not a git repository then ${@srctree_hash_files(d)} will run "git
rev-parse --git-dir" and detect poky directory as git-dir and run "'git', 'add',
'-A', '.'], cwd=s_dir" trying to add srcdir but build dir is in .gitignore and
latest git will fail with "The following paths are ignored by one of your
.gitignore files: build" which will end with "ExpansionError during parsing".

In this commit I added a check if git_dir is the same as git-dir from
TOPDIR (which will detect poky directory) and if yes, then treat srcdir
as non-git sources.

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>