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

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, bdragon
Approved by: re (gjb)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D28897

(cherry picked from commit 0b7472b3d8d2f1e90fade5236b44fd98d8e396c2)
(cherry picked from commit 2c26d77d989abe48c662eeb6f52f7e4c9b81680c)
(cherry picked from commit e77cf2a4ab32a381df3c06d25b8b4f650047c3f2)
(cherry picked from commit e70eb40271512dfbca7cecf823e4b445e3989c2e)