sdk: add base repository fallback

sdk: add base repository fallback

When the build system cannot infer its own repository url then let it fall
back to http://git.openwrt.org/ for the base feed.

The path to openwrt.git is guessed from the VERSION_NUMBER variable:

 "unknown" or "r12345" -> http://git.openwrt.org/openwrt.git
 "15.05.1"             -> http://git.openwrt.org/15.05/openwrt.git

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48212 3c298f89-4303-0410-b956-a3cf2f4a3e73