xtensa: variant-specific code

Allow the variant to provide real code.  Add empty dummy Makefiles for
the existing variants.
Signed-off-by: NJohannes Weiner <jw@emlix.com>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/Makefile

@@ -68,6 +68,9 @@ LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)
 
 head-y		:= arch/xtensa/kernel/head.o
 core-y		+= arch/xtensa/kernel/ arch/xtensa/mm/
+ifneq ($(VARIANT),)
+core-y		+= arch/xtensa/variants/$(VARIANT)/
+endif
 ifneq ($(PLATFORM),)
 core-y		+= arch/xtensa/platforms/$(PLATFORM)/
 endif

arch/xtensa/variants/dc232b/Makefile

+# dc232b Makefile

arch/xtensa/variants/fsf/Makefile

+# fsf Makefile