boot_fat.mk: update to use predefined variables

$(PRODUCT_OUT)/kernel and $(PRODUCT_OUT)/ramdisk.img
are already defined in build as INSTALLED_KERNEL_TARGET
and INSTALLED_RAMDISK_TARGET, so better to use them
in the mk file

Change-Id: I26c49fdbfde922b4af661ef036ce584e4011769f
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/build/tasks/boot_fat.mk b/build/tasks/boot_fat.mk
index e7ddaf4..8a5b5a5 100644
--- a/build/tasks/boot_fat.mk
+++ b/build/tasks/boot_fat.mk
@@ -1,7 +1,7 @@
 ifneq ($(filter hikey%, $(TARGET_DEVICE)),)
 
 ifeq ($(TARGET_BOOTIMAGE_USE_FAT), true)
-$(PRODUCT_OUT)/boot_fat.uefi.img: $(PRODUCT_OUT)/kernel $(PRODUCT_OUT)/hi6220-hikey.dtb $(PRODUCT_OUT)/ramdisk.img
+$(PRODUCT_OUT)/boot_fat.uefi.img: $(INSTALLED_KERNEL_TARGET) $(INSTALLED_RAMDISK_TARGET) $(PRODUCT_OUT)/hi6220-hikey.dtb
 # $@ is referring to $(PRODUCT_OUT)/boot_fat.uefi.img
 	dd if=/dev/zero of=$@ bs=512 count=98304
 	mkfs.fat -n "boot" $@