bsdinstall/zfsboot: Enable new UEFI+GELI support

bsdinstall/zfsboot: Enable new UEFI+GELI support

After r336252 it is no longer necessary to have a separate bootpool when
booting from an encrypted disk with UEFI.

This change also switches the EFI System Partition contents from
the 800 KB boot1.efifat to a new 200 MB filesystem created with newfs_msdos
and uses loader.efi directly, instead of boot1.efi.

PR:		228916
Reviewed by:	dteske
MFC after:	1 month
Relnotes:	yes
Sponsored by:	Klara Systems
Differential Revision:	https://reviews.freebsd.org/D12315