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

target/sdk/Makefile

@@ -37,6 +37,7 @@ SDK_DIRS = \
 BASE_FEED:=$(shell git config --get remote.origin.url 2>/dev/null | sed -ne 's/^/src-git base /p')
 BASE_FEED:=$(if $(BASE_FEED),$(BASE_FEED),$(shell cd $(TOPDIR); LC_ALL=C git svn info 2>/dev/null | sed -ne 's/^URL: /src-gitsvn base /p'))
 BASE_FEED:=$(if $(BASE_FEED),$(BASE_FEED),$(shell cd $(TOPDIR); LC_ALL=C svn info 2>/dev/null | sed -ne 's/^URL: /src-svn base /p'))
+BASE_FEED:=$(if $(BASE_FEED),$(BASE_FEED),src-git base https://git.openwrt.org/$(filter-out unknown/ r%/,$(subst $(space),.,$(wordlist 1,2,$(subst .,$(space),$(VERSION_NUMBER))))/)openwrt.git)
 
 KDIR_BASE = $(patsubst $(TOPDIR)/%,%,$(LINUX_DIR))