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
diff --git a/core/Makefile b/core/Makefile
index 0d83b81..5096e66 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1088,6 +1088,7 @@
 # Remove when b/63676296 is resolved.
 $(error Prebuilt bootimage is only supported for AB targets)
 endif
+INSTALLED_BOOTIMAGE_TARGET := $(PRODUCT_OUT)/boot.img
 $(eval $(call copy-one-file,$(BOARD_PREBUILT_BOOTIMAGE),$(INSTALLED_BOOTIMAGE_TARGET)))
 else # BOARD_PREBUILT_BOOTIMAGE not defined
 INSTALLED_BOOTIMAGE_TARGET :=