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>
1 file changed