RISC-V release confs

Add two release flavors for RISC-V. First, the traditional "big-iron"
images, capable of generating distribution sets and VM images. Installer
images won't be built yet, but can be trivially enabled in the future
with the addition of riscv/make-memstick.sh.

Second, a GENERICSD embedded image. I've opted for this instead of
board-specific SD card images as it allows users to just dd the u-boot
they want. The RISC-V hardware ecosystem is still young, so a
configuration for e.g. the new PolarFire SoC Icicle Kit would likely see
very few users.

Reviewed by: gjb
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27045