EC2: Default to "uefi-preferred" boot mode

EC2: Default to "uefi-preferred" boot mode

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
Reviewed by:	delphij, imp
MFC after:	1 week
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D40470