Add prebuilt init_boot.img to PREBUILT_IMAGES/

It was incorrectly being added to IMAGES/

Bug: 203698939
Test: m
Change-Id: I3c4ae46388a7f043c7bb22b78795277868422a01
diff --git a/core/Makefile b/core/Makefile
index 4c1ae51..6006eed 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -5330,8 +5330,8 @@
 	$(hide) cp $(INSTALLED_SYSTEM_EXTIMAGE_TARGET) $(zip_root)/IMAGES/
 endif
 ifdef BOARD_PREBUILT_INIT_BOOT_IMAGE
-	$(hide) mkdir -p $(zip_root)/IMAGES
-	$(hide) cp $(INSTALLED_INIT_BOOT_IMAGE_TARGET) $(zip_root)/IMAGES/
+	$(hide) mkdir -p $(zip_root)/PREBUILT_IMAGES
+	$(hide) cp $(INSTALLED_INIT_BOOT_IMAGE_TARGET) $(zip_root)/PREBUILT_IMAGES/
 endif
 ifndef BOARD_PREBUILT_BOOTIMAGE
 ifneq (,$(INTERNAL_PREBUILT_BOOTIMAGE) $(filter true,$(BOARD_COPY_BOOT_IMAGE_TO_TARGET_FILES)))