Rework UEFI ESP generation

Currently, the installer uses pre-created 800KB FAT12 filesystems that
it dd's onto the ESP partition.
This changeset improves that by having the installer generate a FAT32
filesystem directly onto the ESP using newfs_msdos and then copying
loader.efi into /EFI/freebsd.
For live installs it then runs efibootmgr to add a FreeBSD boot entry
in the BIOS.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D17947