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

release/riscv/GENERICSD.conf

@@ -0,0 +1,16 @@
+#!/bin/sh
+#
+# $FreeBSD$
+#
+
+EMBEDDED_TARGET_ARCH="riscv64"
+EMBEDDED_TARGET="riscv"
+EMBEDDEDBUILD=1
+FAT_SIZE="54m -b 1m"
+FAT_TYPE="16"
+IMAGE_SIZE="3072M"
+KERNEL="GENERIC"
+MD_ARGS="-x 63 -y 255"
+NODOC=1
+PART_SCHEME="GPT"
+export BOARDNAME="GENERICSD"

release/riscv/riscv64.conf

@@ -0,0 +1,8 @@
+#!/bin/sh
+#
+# $FreeBSD$
+#
+
+TARGET="riscv"
+TARGET_ARCH="riscv64"
+KERNEL="GENERIC"