Add macro to support prebuilt vendor_vboot.img

There are marcos support to link prebuilt images and generate AVB images
in "make"/"make dist" flow. Currently defined marcos only cover boot.img
/init_boot.img/vendor.img and so on, missing the vendor_boot.img.
Appending a marco for prebuilt vendor_boot.img to work like other
prebuilt images.

Test: manual
Build pass in "make" and "make dist" flow and boot up with AVB

Change-Id: I7b296f82e57e8f71a0757d872d35ac27a8d2ecbe
Signed-off-by: chunhui dai <chunhuid@amazon.com>