Revert "build: Handle custom boot images properly"

We don't want to force this since we need to be able to
re-sign boot images with proper keys, which requires rebuilding
of the images.

This also has an error in it from merging (dropped _MK).

This reverts commit 22913de.

Change-Id: Id1353e9883c3f24bbc10e685f43bede779430025

Author: Brint E. Kriebel

core/Makefile

@@ -1536,12 +1536,6 @@ 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, \