Handling source/source-check/external-deps together doesn't work, as
wget will then download the sources every time make source is called
even if it's available in DL_DIR
Instead detect source-check/external-deps from MAKELEVEL.
Support downloadable package patches in source-check/external-deps
while we're at it.
 
 # Retrieve and unpack the archive
 $(BUILD_DIR)/%/.stamp_downloaded:
-# support make source/source-check/external-deps
-ifneq ($(filter source,$(MAKECMDGOALS)),)
+# support make source-check/external-deps
+ifeq ($(MAKELEVEL),1)
        $(WGET) -P $(DL_DIR) $($(PKG)_SITE)/$($(PKG)_SOURCE)
+       $(if $($(PKG)_PATCH),$(WGET) -P $(DL_DIR) $($(PKG)_SITE)/$($(PKG)_PATCH))
 else
        $(call MESSAGE,"Downloading")
 ifneq ($(strip $(subst ",,$(BR2_PRIMARY_SITE))),)