bsdinstall: riscv-specific tweaks

Make the installer more useful, by allowing it to create a bootable
installation. Also, enable the menu option for ZFS-on-root.

Like arm64, RISC-V boots by UEFI only, so arm64's partedit
implementation is renamed and shared among the two platforms.

Reviewed by: gjb
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D28180