package/Makefile: move source tar creation to a function

Split source tarball creation from rpm-pkg target
so it can be used from deb-pkg target as well. As
added bonus, we can now pretty print TAR the name of
tarball created in quiet mode

This patch prepares the groundwork for deb-pkg source
package adding bit.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.com>