package: add support for post-download hooks

This will be needed for the Linux kernel package, which needs to
download patches in a custom way.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/Makefile.package.in

@@ -234,6 +234,7 @@ ifeq ($(DL_MODE),DOWNLOAD)
 endif
 	$(if $($(PKG)_SOURCE),$(call DOWNLOAD,$($(PKG)_SITE),$($(PKG)_SOURCE)))
 	$(if $($(PKG)_PATCH),$(call DOWNLOAD,$($(PKG)_SITE),$($(PKG)_PATCH)))
+	$(foreach hook,$($(PKG)_POST_DOWNLOAD_HOOKS),$(call $(hook))$(sep))
 ifeq ($(DL_MODE),DOWNLOAD)
 	$(Q)mkdir -p $(@D)
 	$(Q)touch $@
@@ -437,6 +438,7 @@ $(2)_EXTRACT_CMDS ?= \
 	$(TAR) $(TAR_STRIP_COMPONENTS)=1 -C $$($(2)_DIR) $(TAR_OPTIONS) -)
 
 # post-steps hooks
+$(2)_POST_DOWNLOAD_HOOKS        ?=
 $(2)_POST_EXTRACT_HOOKS         ?=
 $(2)_POST_PATCH_HOOKS           ?=
 $(2)_PRE_CONFIGURE_HOOKS        ?=