x86/boot: fix reloc.S build dependencies

reloc.S is not rebuild if header included
in reloc.c is updated. Fix this issue.

Additionally, remove reloc.S build dependency
on head.S because anything from reloc.S does
not depend on head.S.

Add reloc.c dependency to reloc.o build rule for consistency.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/Makefile

@@ -1,8 +1,8 @@
 obj-bin-y += head.o
 
+RELOC_DEPS = $(BASEDIR)/include/asm-x86/config.h $(BASEDIR)/include/xen/multiboot.h
+
 head.o: reloc.S
 
-%.S: %.c
-	$(MAKE) -f build32.mk $@
-
-reloc.S: head.S
+reloc.S: reloc.c $(RELOC_DEPS)
+	$(MAKE) -f build32.mk $@ RELOC_DEPS="$(RELOC_DEPS)"

xen/arch/x86/boot/build32.mk

@@ -30,5 +30,6 @@ CFLAGS := $(filter-out -flto,$(CFLAGS))
 			esac; \
 		done
 
-reloc.o: $(BASEDIR)/include/asm-x86/config.h
+reloc.o: reloc.c $(RELOC_DEPS)
+
 .PRECIOUS: %.bin %.lnk