RISC-V release confs

[mhorne]

Summary

Add two release flavors for RISC-V. First, the traditional "big-iron" images,
capable of generating release tarballs 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 HiFive Unleashed would likely see very few users.

Test Plan

Built and booted several times using both release.sh and bare make release.

Generated images available at https://people.freebsd.org/~mhorne/riscv_images/

Can be booted with sysutils/opensbi and sysutils/u-boot-qemu-riscv64 as follows:

$ qemu-system-riscv64 -smp 2 -m 2048m -machine virt -nographic \
      -bios /usr/local/share/opensbi/lp64/generic/firmware/fw_jump.elf \
      -kernel /usr/local/share/u-boot/u-boot-qemu-riscv64/u-boot.bin \
      -drive file=FreeBSD-13.0-CURRENT-riscv-riscv64.raw,format=raw,id=hd0 \
      -device virtio-blk-device,drive=hd0

and for tap networking add:

-netdev tap,ifname=tap0,script=no,id=net0 -device virtio-net-device,netdev=net0