Don't copy recovery.img to BOOTABLE_IMAGES if it doesn't exist

If BOARD_USES_RECOVERY_AS_BOOT is set to true, recovery.img is
not built.

If BOARD_CUSTOM_BOOTIMG is set, INSTALLED_RECOVERYIMAGE_TARGET
gets copied to BOOTABLE_IMAGES.

If both are set, the following error is produced.

acp: missing destination file

Since it is possible to have both BOARD_USES_RECOVERY_AS_BOOT and
BOARD_CUSTOM_BOOTIMG set, guard the copy operation so that it does
not try to copy a missing recovery.img.

Change-Id: Iae24e31a586e98955807d213df3dbaa0a2f13bae
diff --git a/core/Makefile b/core/Makefile
index c834338..6d3fcf2 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -6726,8 +6726,10 @@
 	@# Prebuilt boot images
 	$(hide) mkdir -p $(zip_root)/BOOTABLE_IMAGES
 	$(hide) $(ACP) $(INSTALLED_BOOTIMAGE_TARGET) $(zip_root)/BOOTABLE_IMAGES/
+ifneq ($(INSTALLED_RECOVERYIMAGE_TARGET),)
 	$(hide) $(ACP) $(INSTALLED_RECOVERYIMAGE_TARGET) $(zip_root)/BOOTABLE_IMAGES/
 endif
+endif
 ifdef BUILDING_SYSTEM_IMAGE
 	@# Contents of the system image
 ifneq ($(SOONG_DEFINED_SYSTEM_IMAGE_PATH),)