ipkg-build: use deterministic timestamps in ipk tarballs

As a follow-up to r46026, this commit sets the modification
times of files inside the tarballs to deterministic values.
It uses the date of the last git or svn commit and falls
back to the old behavior if those are not available.

Signed-off-by: Reiner Herrmann <reiner@reiner-h.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46360 3c298f89-4303-0410-b956-a3cf2f4a3e73