[images] Have bootserver mexec a kernel for arm64.

Although we wish to minimize the usage of mexec (ZX-2069), the infrastructure
currently requires it for vim2 lifecycle management. (`fastboot continue`
does not continue back to fuchsia after paving and rebooting in the case we
do not mexec a kernel.)

Test: mexec-ing zircon.zbi works for zircon on a vim2.
Bug: IN-892
Change-Id: Ifb2f62ef858535e1f06a50d118b56c37d8394f9e
1 file changed