fwk: Allow building without .git

Allow building of scp-firmware when there is not Git information
available in scp-firmware source tree. This situation occurs with
distribution or development environments that use a scp-firmware
source tree tarball that do not content Git information.

When there is no .git entry at scp-firmware root directory, the git
describe command returns an error message (below), preventing to build
the sources:
fatal: not a git repository (or any parent up to mount point /mnt)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

Change-Id: Ia8e620f74054066e1c7b2336367e8ef71a60a0fe
Signed-off-by: Christophe Priouzeau <christophe.priouzeau@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>