[ARM] Only decend into mach-* if $(MACHINE) is defined.

Recent kbuild changes means that make clean fails on ARM because we
try to decend into "arch/arm/mach-" which doesn't exist.  This cset
fixes this by excluding this directory from core-y if $(MACHINE) is
undefined.

arch/arm/Makefile

@@ -179,8 +179,10 @@ FASTFPE_OBJ	:=$(FASTFPE)/
 endif
 
 # If we have a machine-specific directory, then include it in the build.
-core-y				+= arch/arm/mach-$(MACHINE)/ \
-				   arch/arm/kernel/ arch/arm/mm/
+ifneq ($(MACHINE),)
+core-y				+= arch/arm/mach-$(MACHINE)/
+endif
+core-y				+= arch/arm/kernel/ arch/arm/mm/
 core-$(CONFIG_FPE_NWFPE)	+= arch/arm/nwfpe/
 core-$(CONFIG_FPE_FASTFPE)	+= $(FASTFPE_OBJ)