Build: Fix issue when build directory is not in TF-M

Building out of te TF-M repo folder causes an error:
fatal: not a git repository (or any parent up to mount point /)
reported from cmake/version.cmake.

This patch fixes this by setting the WORKING_DIRECTORY.

Change-Id: Ideebbd0d58811038b9300e8466fd5d7bac9fc821
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
1 file changed