Set INSTALLED_BOOTIMAGE_TARGET in case of TARGET_NO_KERNEL is true and

BOARD_PREBUILT_BOOTIMAGE is set.

Based on current Makefile INSTALLED_BOOTIMAGE_TARGET always kept as
empty if TARGET_NO_KERNEL is true, so build fails with empty out path.
This change is to set up INSTALLED_BOOTIMAGE_TARGET with
$(PRODUCT_OUT)/boot.img in case of TARGET_NO_KERNEL is true and
BOARD_PREBUILT_BOOTIMAGE is defined.

Bug: 135648566
Test: m -j passed
Change-Id: I9658f215c2e2cc5d87040d5085521ef44722c4ef