xen/arm: build: add missed dependency for head.S

When we update the header files that had been included in head.S.
The build system would not re-compile the head.S. Because in the
build rules, the dependencies are setting to .*.d (eg. DEPS = .*.d)
files in the same folder as Makefile.

But head.S is very special, it was used by the Makefile in the parent
folder: "ALL_OBJS := $(TARGET_SUBARCH)/head.o".

In this case, the build system could not find the dependency in DEPS.
When we update the header files, the build system is unware of this
update. If we re-build the Xen without doing make clean or touching
the head.S, the build system will not recompile the head.S.

Signed-off-by: Wei Chen <Wei.Chen@linaro.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
master commit: 9ea859deda

xen/Rules.mk

@@ -92,9 +92,10 @@ LDFLAGS += $(LDFLAGS-y)
 
 include $(BASEDIR)/arch/$(TARGET_ARCH)/Rules.mk
 
+DEPS = .*.d
+
 include Makefile
 
-DEPS = .*.d
 define gendep
     ifneq ($(1),$(subst /,:,$(1)))
         DEPS += $(dir $(1)).$(notdir $(1)).d

xen/arch/arm/Makefile

@@ -51,6 +51,8 @@ endif
 
 ALL_OBJS := $(TARGET_SUBARCH)/head.o $(ALL_OBJS)
 
+DEPS += $(TARGET_SUBARCH)/.head.o.d
+
 $(TARGET): $(TARGET)-syms $(TARGET).axf
 	$(OBJCOPY) -O binary -S $< $@
 ifeq ($(CONFIG_ARM_64),y)