调整脚本,使得能够创建riscv的磁盘镜像,并引导进入riscv下的grub (#450)

* 安装musl toolchain以及riscv相关的工具链

* 调整脚本,使得能够创建riscv的磁盘镜像,并引导进入riscv下的grub

```shell
export ARCH=riscv64
make write_diskimage
make qemu
```

即可在serial_opt.txt看到进入grub的提示信息

tools/umount_virt_disk.sh

@@ -5,6 +5,13 @@ if [ ! $uid == "0" ];then
  exit
 fi
 
+if [ ! ${ARCH} ];then
+ echo "请设置ARCH环境变量"
+ exit
+fi
+
+DISK_NAME=disk-${ARCH}.img
+
 LOOP_DEVICE=$(lsblk | grep disk_mount|sed 's/.*\(loop[0-9]*\)p1.*/\1/1g'|awk 'END{print $0}')
 
 umount -f ../bin/disk_mount/