externalsrc: fix lookup for .gitmodules

Commit 0533edac27 broke
bitbake parsing when bitbake is executed from directory with existing .gitmodules
and the recipe in externalsrc does not have .gitmodules

The check needs to search for .gitmodules in sources path, not cwd.

iParsing recipes...ERROR: ExpansionError during parsing <path to recipe>
...
bb.data_smart.ExpansionError: Failure expanding variable do_compile[file-checksums], expression was ${@srctree_hash_files(d)} which triggered exception CalledProcessError: Command '['git', 'config', '--file', '.gitmodules', '--get-regexp', 'path']' returned non-zero exit status 1.

(From OE-Core rev: 1f3b39c4d6828940934a341e9f8b73214dc67fdf)

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 66ff3d1f65cd2e7f5319e98fa41f47a59b714c72)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -229,7 +229,7 @@ def srctree_hash_files(d, srcdir=None):
             env['GIT_INDEX_FILE'] = tmp_index.name
             subprocess.check_output(['git', 'add', '-A', '.'], cwd=s_dir, env=env)
             git_sha1 = subprocess.check_output(['git', 'write-tree'], cwd=s_dir, env=env).decode("utf-8")
-            if os.path.exists(".gitmodules"):
+            if os.path.exists(os.path.join(s_dir, ".gitmodules")):
                 submodule_helper = subprocess.check_output(["git", "config", "--file", ".gitmodules", "--get-regexp", "path"], cwd=s_dir, env=env).decode("utf-8")
                 for line in submodule_helper.splitlines():
                     module_dir = os.path.join(s_dir, line.rsplit(maxsplit=1)[1])