nanobsd: Set a GPT label for EFI and cfg partitions

Use efiboot0 for the EFI partition, the nomenclature efiboot + #index
was taken from bsdinstall (zfsboot).

Use cfg for the cfg partition. Poudriere firmware images are already
using this label.

PR: 278480
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1187