fastboot driver: repack vendor boot ramdisk

When a user issues `fastboot flash vendor_boot:foo ramdisk.img`, the fastboot driver
fetches the vendor_boot image from the device,
determines if `foo` is a valid vendor ramdisk fragment,
repacks a new vendor boot image, then
flash the vendor boot image back.
This requires vendor boot header V4.

As a convinent alias, `fastboot flash vendor_boot:default ramdisk.img`
flashes the whole vendor ramdisk image. This works on vendor boot header
V3 & 4.

Fixes: 173654501
Test: pass

Change-Id: I42b2483a736ea8aa9fd9372b960502a642934cdc