scripts/mkimg.minirootfs.sh: add support for riscv64

scripts/mkimg.minirootfs.sh

@@ -20,6 +20,6 @@ profile_minirootfs() {
 		and minimal chroots."
 	image_ext=tar.gz
 	output_format=rootfs
-	arch="x86 x86_64 armhf armv7 aarch64 ppc64le s390x mips64"
+	arch="x86 x86_64 armhf armv7 aarch64 ppc64le s390x mips64 riscv64"
 	rootfs_apks="busybox alpine-baselayout alpine-keys apk-tools libc-utils"
 }