packages: add PKG_URL variable

Using PKG_URL one may set the URL for all sub packages, which is usually
shared anyway. Future packages should only use PKG_URL instead of adding
it per sub-package.

Signed-off-by: Paul Spooren <mail@aparcar.org>

include/package-defaults.mk

@@ -49,7 +49,7 @@ define Package/Default
   KCONFIG:=
   BUILDONLY:=
   HIDDEN:=
-  URL:=
+  URL:=$(PKG_URL)
   VARIANT:=
   DEFAULT_VARIANT:=
   USERID:=