[images] Use multiboot.bin as "kernel" for vboot (CrOS)

Also stop producing vboot images for arm64, which doesn't use them.

Bug: BLD-269 #done
Test: CQ & manual booting of Pixelbook with new fuchsia.vboot
Change-Id: I654900260b402dfe463932eaa07d6fdb9688c2db
3 files changed