release.sh: add support for RISC-V embedded builds

Since the few existing RISC-V hardware platforms are single board
computers, we can piggyback off of arm/arm64's embedded build support
for generating SD card images.

I don't see a pressing need to change the naming in this file at this
time.

Reviewed by: gjb, manu
Differential Revision: https://reviews.freebsd.org/D27043