Allow makefs to properly tag UEFI El Torito boot images. Use them in amd64 ISOs.

UEFI booting requires an EFI System Partition (ESP). On most storage devices
this will be in a specific partition type. To allow booting from CD/ISO
filesystems, UEFI will look for an ESP in the form of a FAT filesystem image
embedded in the image. Historically FreeBSD has added one of these to its
amd64 ISO images but marked it as simply another i386 boot image. Luckily for
us most UEFI implementations are rather forgiving and work this out for us.

This change adds the ability to mark a boot image as being a UEFI image. It
also modifies our ISO generation to use this marking for the UEFI image we
embed.

Reported by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed by: emaste, imp
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14809