image: always rebuild kernel loaders

Kernel loaders like the lzma-loader currently don't track changes to
their sources. This can lead to an old version of a loader to be used
when a build tree is not clean between builds.

As the loaders are tiny and the build times are insignificant, simply
force rebuilding them on every build to avoid this problem.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

include/image.mk

@@ -517,7 +517,7 @@ endif
 define Device/Build/compile
   $$(_COMPILE_TARGET): $(KDIR)/$(1)
   $(eval $(call Device/Export,$(KDIR)/$(1)))
-  $(KDIR)/$(1):
+  $(KDIR)/$(1): FORCE
 	$$(call concat_cmd,$(COMPILE/$(1)))
 
 endef