EC2: Allow AMI boot mode to be specified

The default boot method for amd64 AMIs is BIOS, but at AMI creation
time a flag can be set to specify that UEFI should be used instead.
This commit adds a variable AMIBOOTMETHOD which, if set to "UEFI",
causes the appropriate flag to be set during AMI creation.

The only boot method supported by EC2 for arm64 is UEFI.

The names of AMIs are also amended to include the boot method; they
now look like "FreeBSD 14.0-CURRENT-amd64-20210915 UEFI".

MFC after: 1 week
Sponsored by: https://www.patreon.com/cperciva