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>

SVN-Revision: 48212