Revert "Revert "build: Handle custom boot images properly""

This is still needed, per this discussion here:
http://review.cyanogenmod.org/#/c/76919/

This reverts commit 78d216b.

Change-Id: If494c2c6468185bc0e10cdb92847a9f8429b79c4

Author: Brint E. Kriebel

core/Makefile

@@ -1536,6 +1536,12 @@ endif
 	@# Contents of the data image
 	$(hide) $(call package_files-copy-root, \
 		$(TARGET_OUT_DATA),$(zip_root)/DATA)
+ifdef BOARD_CUSTOM_BOOTIMG
+	@# Prebuilt boot images
+	$(hide) mkdir -p $(zip_root)/BOOTABLE_IMAGES
+	$(hide) $(ACP) $(INSTALLED_BOOTIMAGE_TARGET) $(zip_root)/BOOTABLE_IMAGES/
+	$(hide) $(ACP) $(INSTALLED_RECOVERYIMAGE_TARGET) $(zip_root)/BOOTABLE_IMAGES/
+endif
 ifdef BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE
 	@# Contents of the vendor image
 	$(hide) $(call package_files-copy-root, \