build: allow specifying branch to git clone

If you're testing build packaging inside of OpenWRT at the same time
that you're making changes to a package upstream, and you want to be
sure that they'll work together with no changes after being merged,
it's handy to be able to (temporarily) specify the branch you're
working in upstream as the branch to clone and build.

Obviously you'd drop the look-aside branch from the OpenWRT
packaging once the upstream changes you're testing have been
merged, and go back to whatever the default branch (usually "main"
or "master") for that repository is.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

include/download.mk

@@ -311,6 +311,7 @@ define Download/default
   URL_FILE:=$(PKG_SOURCE_URL_FILE)
   SUBDIR:=$(PKG_SOURCE_SUBDIR)
   PROTO:=$(PKG_SOURCE_PROTO)
+  OPTS:=$(if $(PKG_SOURCE_BRANCH),-b "$(PKG_SOURCE_BRANCH)")
   SUBMODULES:=$(PKG_SOURCE_SUBMODULES)
   $(if $(PKG_SOURCE_MIRROR),MIRROR:=$(filter 1,$(PKG_MIRROR)))
   $(if $(PKG_MIRROR_MD5SUM),MIRROR_MD5SUM:=$(PKG_MIRROR_MD5SUM))