package: remove .git files from images

If you have your ./files/ directory stored in a git repository,
the .git will be included into images using precious space.
This patch removes .git directories before packing images.

Signed-off-by: Joerg Jungermann <jj@borkum.net>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 49180