Add support for building vendor_ramdisk.img

Allow BoardConfig.mk to set:
BOARD_BUILD_VENDOR_RAMDISK_IMAGE := true

to build:
out/target/product/<name>/vendor_ramdisk.img

This is done to support GKI device kernel development.

Kernel developers build vmlinux + DTB + GKI kernel modules locally
and then use kernel build tools to output:

* vmlinux + ramdisk.img => boot.img
* GKI kernel modules + DTB + vendor_ramdisk.img => vendor_boot.img

In other words, kernel developers use ramdisk.img and vendor_ramdisk.img
as prebuilts for building flashable boot.img and vendor_boot.img
directly from the kernel development environment.

Test: make vendorramdisk with BOARD_BUILD_VENDOR_RAMDISK_IMAGE := true
Test: make vendorramdisk with BOARD_BUILD_VENDOR_RAMDISK_IMAGE omitted
Change-Id: Id67839887b6bf608f4a5f13384c551c12ee9fdbd
diff --git a/core/Makefile b/core/Makefile
index c8b8ae9..a91b40d 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -990,6 +990,13 @@
 $(INTERNAL_VENDOR_RAMDISK_TARGET): $(MKBOOTFS) $(INTERNAL_VENDOR_RAMDISK_FILES) | $(COMPRESSION_COMMAND_DEPS)
 	$(MKBOOTFS) -d $(TARGET_OUT) $(TARGET_VENDOR_RAMDISK_OUT) $(PRIVATE_ADDITIONAL_DIR) | $(COMPRESSION_COMMAND) > $@
 
+ifeq (true,$(BOARD_BUILD_VENDOR_RAMDISK_IMAGE))
+INSTALLED_VENDOR_RAMDISK_TARGET := $(PRODUCT_OUT)/vendor_ramdisk.img
+$(INSTALLED_VENDOR_RAMDISK_TARGET): $(INTERNAL_VENDOR_RAMDISK_TARGET)
+	$(call pretty,"Target vendor ram disk: $@")
+	$(copy-file-to-target)
+endif
+
 INSTALLED_FILES_FILE_VENDOR_RAMDISK := $(PRODUCT_OUT)/installed-files-vendor-ramdisk.txt
 INSTALLED_FILES_JSON_VENDOR_RAMDISK := $(INSTALLED_FILES_FILE_VENDOR_RAMDISK:.txt=.json)
 $(INSTALLED_FILES_FILE_VENDOR_RAMDISK): .KATI_IMPLICIT_OUTPUTS := $(INSTALLED_FILES_JSON_VENDOR_RAMDISK)