bsdinstall: Add loader.efi to all ESPs we create

For proper redundancy, add copies of loader.efi to each of the ESPs we
create when we create multi-volume ZFS datasets. zfsboot creates a list
of secondary ESPs, while bootpart doesn't create any (it's the UFS
partitioning tool) because we don't supporg UFS over gmirror. The
primary ESP is mounted and is what we use efibootmgr to boot from. The
redundant copies allow the system to boot if the primary disks fails.

Sponsored by: Netflix
MFC After: 2 days
PR: 208802
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D52780