Add GPT PREP-boot type to mkimg(1) from geom_gpt.

This partition type can be used to boot some PowerKVM VMs. We don't
support it well because of some limitations in SLOF, but it's worth at
least have feature parity in geom and mkimg.

(cherry picked from commit f7aebfbe5c0ab63dbb3d134d143ccff2c2b278bb)