include/download.mk: Allow specify DownloadMethod specific options

This is going to be used to migrate the hand rolled git clone for the kernel
into using the git download method. The kernel uses custom options that we may
have to pass down.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

include/download.mk

@@ -93,7 +93,7 @@ define DownloadMethod/git
 		cd $(TMP_DIR)/dl && \
 		rm -rf $(SUBDIR) && \
 		[ \! -d $(SUBDIR) ] && \
-		git clone $(URL) $(SUBDIR) && \
+		git clone $(OPTS) $(URL) $(SUBDIR) && \
 		(cd $(SUBDIR) && git checkout $(VERSION) && \
 		git submodule update --init --recursive) && \
 		echo "Packing checkout..." && \
@@ -172,6 +172,7 @@ define Download/Defaults
   MIRROR:=1
   MIRROR_MD5SUM:=x
   VERSION:=
+  OPTS:=
 endef
 
 define Download