EC2: Build and upload AMIs for both UFS and ZFS

The pre-existing "ec2ami" target builds and uploads a single AMI
(with filesystem determined by ${VMFS}) as before; a new "ec2amis"
target does both UFS and ZFS.

Reviewed by: gjb
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41343