EC2: Default to UEFI booting

EC2: Default to UEFI booting

This reduces the FreeBSD boot time by approximately 5 seconds,
roughly equally divided betwenn two factors:
* Disk I/O is faster in the EFI loader since it can perform larger
I/Os.  (The BIOS loader is limited due to the use of bounce buffers
in sub-1M memory.)
* The EFI console is much faster than the VGA console.

Note however that not all EC2 instance types support UEFI; as a
general rule the newer instances (based on Amazon's "Nitro" platform)
support UEFI but the older instances (based on Xen) do not.

X-MFC:	TBD based on tradeoff between performance and compatibility
Relnotes:	yes
Sponsored by:	https://www.patreon.com/cperciva