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
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