cmake: Support ZEPHYR_BASE to be a git submodule

In 91709778 build-time version header generation was added. The test
for .git assumes this file to be a directory. In the case of git
submodules, .git is a regular file that in its contents points to
the actual git database for the submodule. This is a way to have
symlink like behaviour even on file systems that do not support
themselves support symlinks.

This consults git as to what the correct git database directory is,
in case the .git file is indeed a regular file, and adjusts the
git_dependency variable accordingly.

Fixes #43503

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>