[PATCH] Arm: fix build with CONFIG_DTB_FILE set

[Jan Beulich <jbeulich@xxxxxxxx>]

Recent changes no longer allow modification of AFLAGS. The needed
conversion was apparently missed in 2740d96efdd3 ("xen/build: have the
root Makefile generates the CFLAGS").

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/arm/Makefile
+++ b/xen/arch/arm/Makefile
@@ -68,7 +68,7 @@ extra-y += $(TARGET_SUBARCH)/head.o
 
 ifdef CONFIG_DTB_FILE
 obj-y += dtb.o
-AFLAGS += -DCONFIG_DTB_FILE=\"$(CONFIG_DTB_FILE)\"
+AFLAGS-y += -DCONFIG_DTB_FILE=\"$(CONFIG_DTB_FILE)\"
 endif
 
 ALL_OBJS := $(TARGET_SUBARCH)/head.o $(ALL_OBJS)