riscv: allow building virtual machine images

RISC-V has the same booting requirements as arm64 (loader.efi, no legacy
boot options), so generated images for both architectures have the same
partition layout.

Reviewed by:	gjb
Differential Revision:	https://reviews.freebsd.org/D27044

release/tools/vmimage.subr

@@ -30,7 +30,7 @@ write_partition_layout() {
 				   -p freebsd-boot/bootfs:=${BOOTFILES}/i386/gptboot/gptboot"
 			ROOTFSPART="-p freebsd-ufs/rootfs:=${VMBASE}"
 			;;
-		arm64:aarch64)
+		arm64:aarch64 | riscv:riscv64*)
 			ESP=yes
 			SCHEME=gpt
 			BOOTPARTS=

tools/boot/install-boot.sh

@@ -38,6 +38,7 @@ get_uefi_bootname() {
         arm64) echo bootaa64 ;;
         i386) echo bootia32 ;;
         arm) echo bootarm ;;
+        riscv) echo bootriscv64 ;;
         *) die "machine type $(uname -m) doesn't support UEFI" ;;
     esac
 }