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