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.