Don't use pfx and sfx when not exist because cause formatting errors
like 2.9HASH instead of 2.9+gitAUTOINC+HASH.
(From OE-Core rev: 382833cc7e7603241838e18e429f69da0c2ee4ac)
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>