extmod/vfs: Rewrite path lookup algo to support relative paths from root.

For example, if the current directory is the root dir then this patch
allows one to do uos.listdir('mnt'), where 'mnt' is a valid mount point.
Previous to this patch such a thing would not work, on needed to do
uos.listdir('/mnt') instead.

extmod/vfs.c

@@ -43,33 +43,38 @@
 // Returns MP_VFS_ROOT for root dir (and then path_out is undefined) and
 // MP_VFS_NONE for path not found.
 mp_vfs_mount_t *mp_vfs_lookup_path(const char *path, const char **path_out) {
-    if (path[0] == '/' && path[1] == 0) {
-        return MP_VFS_ROOT;
-    } else if (MP_STATE_VM(vfs_cur) == MP_VFS_ROOT) {
-        // in root dir
-        if (path[0] == 0) {
-            return MP_VFS_ROOT;
+    if (*path == '/' || MP_STATE_VM(vfs_cur) == MP_VFS_ROOT) {
+        // an absolute path, or the current volume is root, so search root dir
+        bool is_abs = 0;
+        if (*path == '/') {
+            ++path;
+            is_abs = 1;
         }
-    } else if (*path != '/') {
-        // a relative path within a mounted device
-        *path_out = path;
-        return MP_STATE_VM(vfs_cur);
-    }
-
-    for (mp_vfs_mount_t *vfs = MP_STATE_VM(vfs_mount_table); vfs != NULL; vfs = vfs->next) {
-        if (strncmp(path, vfs->str, vfs->len) == 0) {
-            if (path[vfs->len] == '/') {
-                *path_out = path + vfs->len;
-                return vfs;
-            } else if (path[vfs->len] == '\0') {
-                *path_out = "/";
-                return vfs;
+        for (mp_vfs_mount_t *vfs = MP_STATE_VM(vfs_mount_table); vfs != NULL; vfs = vfs->next) {
+            size_t len = vfs->len - 1;
+            if (strncmp(path, vfs->str + 1, len) == 0) {
+                if (path[len] == '/') {
+                    *path_out = path + len;
+                    return vfs;
+                } else if (path[len] == '\0') {
+                    *path_out = "/";
+                    return vfs;
+                }
             }
         }
+        if (*path == '\0') {
+            // path was "" or "/" so return virtual root
+            return MP_VFS_ROOT;
+        }
+        if (is_abs) {
+            // path began with / and was not found
+            return MP_VFS_NONE;
+        }
     }
 
-    // mount point not found
-    return MP_VFS_NONE;
+    // a relative path within a mounted device
+    *path_out = path;
+    return MP_STATE_VM(vfs_cur);
 }
 
 // Version of mp_vfs_lookup_path that takes and returns uPy string objects.