package: add a PRE_PATCH_HOOKS hook point

This will be useful to integrate Xenomai and RTAI into Buildroot, so
that they can patch the kernel before the normal patching process
starts.

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

package/Makefile.package.in

@@ -259,6 +259,7 @@ $(BUILD_DIR)/%/.stamp_extracted:
 $(BUILD_DIR)/%/.stamp_patched: NAMEVER = $(RAWNAME)-$($(PKG)_VERSION)
 $(BUILD_DIR)/%/.stamp_patched:
 	@$(call MESSAGE,"Patching $($(PKG)_DIR_PREFIX)/$(RAWNAME)")
+	$(foreach hook,$($(PKG)_PRE_PATCH_HOOKS),$(call $(hook))$(sep))
 	$(if $($(PKG)_PATCH),support/scripts/apply-patches.sh $(@D) $(DL_DIR) $($(PKG)_PATCH))
 	$(Q)( \
 	if test -d $($(PKG)_DIR_PREFIX)/$(RAWNAME); then \
@@ -443,6 +444,7 @@ $(2)_EXTRACT_CMDS ?= \
 # post-steps hooks
 $(2)_POST_DOWNLOAD_HOOKS        ?=
 $(2)_POST_EXTRACT_HOOKS         ?=
+$(2)_PRE_PATCH_HOOKS            ?=
 $(2)_POST_PATCH_HOOKS           ?=
 $(2)_PRE_CONFIGURE_HOOKS        ?=
 $(2)_POST_CONFIGURE_HOOKS       ?=