EC2: Add AMI Builder AMI building

Starting in 2015 I have published "AMI Builder AMIs" for FreeBSD/EC2:
These boot into a memory disk, extract a "clean" copy of FreeBSD onto
the root disk, mount it

EC2: Add AMI Builder AMI building

Starting in 2015 I have published "AMI Builder AMIs" for FreeBSD/EC2:
These boot into a memory disk, extract a "clean" copy of FreeBSD onto
the root disk, mount it at /mnt, and allow the user to SSH in to make
customizations before creating a new AMI from the "running" instance
(in fact, from the FreeBSD installation which is not running but is
mounted on /mnt).

This provides a much cleaner mechanism for building customized FreeBSD
AMIs than the traditional Linux approach of "launch an EC2 instance,
SSH in and configure it, then try to wipe logs and credentials before
creating an AMI"; and it's easier than building a customized AMI ab
initio by modifying the FreeBSD release-building code.

This commit brings that functionality into the FreeBSD src tree and
into the collection of images built by the release engineering team:
The EC2 "BUILDER" flavour AMI is essentially a "SMALL" flavour AMI with
a compressed "BASE" flavour disk image, plus an init script which
juggles disks around (rerooting into a memory disk and extracting the
"BASE" image onto disk).

Polished by: bz, emaste
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D49930

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