In EC2, UEFI boots faster than BIOS, but not all amd64 instance types
support UEFI. AMIs need to have their boot mode designated, which
created a dilemma: Faster boots, or wider compatibility?
The recently added "uefi-preferred" option solves this: AMIs can be
marked to use UEFI where it's available, but fall back to BIOS on
instance types which do not support UEFI.
This uses bsdec2-image-upload 1.4.6, which recently landed in the
ports tree.
PR: 265697
MFC after: 1 week
Sponsored by: https://www.patreon.com/cperciva