Add macro for output of 'git describe'

'@GIT_DESCRIPTION@' can now be used as a placeholder for the output of
'git describe'. If git doesn't find any names to describe this
revision '@GIT_DESCRIPTION@' is replaced by the revision ID.

Change-Id: Ic942e827af7207396f3fa8bf341b131a27df8798
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>