Make build support vendor_kernel_boot image

Add vendor_kernel_boot image for vendors whose bootloader support
extra first stage booting kernel modules ramdisks. This benefit
kernel repo to build kernel-artifacts only image without Andorid
artifacts dependency.

Bug: 214409109
Signed-off-by: Lucas Wei <lucaswei@google.com>
Change-Id: If07218b86a7751b3d452a172610af960f5f9ec74