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