Mount the EFI system partition (ESP) on newly-installed systems.

Per hier(7), the ESP will be mounted at /boot/efi. On UFS systems,
any existing ESP will be reused and mounted there; otherwise, a new one
will be made. On ZFS systems, space for an ESP is allocated on all disks
in the root pool, but only the partition actually used to boot is set up
and mounted.

This makes future upgrades of the EFI loader easier (upgrade scripts can
just change /boot/efi) and also greatly simplifies the parts of the
installer involved in initialization of the ESP. It also makes the
installer's behavior correspond to the documentation in hier(7).

Reviewed by: imp, tsoome
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D28897