Update readme and make virtio-fs optional

If -s parameter is empty, virtio-fs is unused.
Add the explanation about virtio-fs in `Usage` of README.md.

Author: MikazukiHikari

README.md

@@ -77,14 +77,14 @@ You can exit the emulator using: \<Ctrl-a x\>. (press Ctrl+A, leave it, afterwar
 ## Usage
 
 ```shell
-./semu -k linux-image [-b dtb-file] [-i initrd-image] [-d disk-image]
+./semu -k linux-image [-b dtb-file] [-i initrd-image] [-d disk-image] [-s mount-folder]
 ```
 
 * `linux-image` is the path to the Linux kernel `Image`.
 * `dtb-file` is optional, as it specifies the user-specified device tree blob.
 * `initrd-image` is optional, as it specifies the user-specified initial RAM disk image.
 * `disk-image` is optional, as it specifies the path of a disk image in ext4 file system for the virtio-blk device.
-
+* `mount-folder` is optional, as it specifies the path of a folder you want to mount in host.
 ## Build Linux kernel image and root file system
 
 An automated build script is provided to compile the RISC-V cross-compiler, Busybox, and Linux kernel from source.

main.c

@@ -536,10 +536,10 @@ static void map_file(char **ram_loc, const char *name)
 
 static void usage(const char *execpath)
 {
-    fprintf(
-        stderr,
-        "Usage: %s -k linux-image [-b dtb] [-i initrd-image] [-d disk-image]\n",
-        execpath);
+    fprintf(stderr,
+            "Usage: %s -k linux-image [-b dtb] [-i initrd-image] [-d "
+            "disk-image] [-s mount-folder]\n",
+            execpath);
 }
 
 static void handle_options(int argc,

virtio-fs.c

@@ -833,6 +833,11 @@ bool virtio_fs_init(virtio_fs_state_t *vfs, char *mtag, char *dir)
 
     vfs->priv = &vfs_configs[vfs_dev_cnt++];
 
+    if (!dir) {
+        /* -s parameter is empty, virtio-fs is unused. */
+        return NULL;
+    }
+
     int dir_fd = open(dir, O_RDONLY);
     if (dir_fd < 0) {
         fprintf(stderr, "Could not open directory: %s\n", dir);