gitconfig: Improve robustness when blobs aren't present

gitconfig: Improve robustness when blobs aren't present

With no blobs present the 'make gitconfig' target failed when
trying to add a file to a directory which doesn't exist.
Only try to deal with blobs if they're around.

Change-Id: I27ed33e2e22bb1571bc73fe55cf45aa1e2310bf1
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10806
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)