scripts: dbglfs3.py: Adopted % as littlefs root dir character

The idea is this is similar to ~ for the home directory, hopefully
simplifying littlefs path parsing in scripts:

- /hi -> hi in root dir
- ./hi -> hi in current dir
- ../hi -> hi in parent dir
- ~/hi -> hi in home dir
- %/hi -> hi in littlefs root dir

Note this only works when standalone:

- ~hi != ~/hi
- %hi != %/hi

And files named % can still be referenced with a ./ prefix:

- ./% -> % in current dir
- %/% -> % in littlefs root dir

---

This is probably overkill for dbglfs3.py, as the arg ordering was
already enough to disambiguate disk path vs mroot address vs littlefs
path, but eventually I think the idea will be useful for more powerful
scripts.

A hypothetical:

  $ mklfs3 cp disk -b4096 -r image_files %/image_files

scripts/dbglfs3.py

@@ -4422,8 +4422,14 @@ def dbg_files(lfs, paths, *,
     # parse all paths first, error if anything is malformed
     dirs = []
     # default paths to the root dir
-    for path in (paths or ['/']):
+    for path in (paths or ['%']):
         try:
+            # skip leading %
+            if path == '%':
+                path = '/'
+            if path.startswith('%/'):
+                path = path[1:]
+            # lookup path
             dir = lfs.pathlookup(path,
                     all=args.get('all'))
         except Lfs3.PathError as e:
@@ -4952,16 +4958,17 @@ if __name__ == "__main__":
     parser.add_argument(
             'mroots',
             nargs='*',
-            type=lambda x: rbydaddr(x) if not x.startswith('/') else x,
+            type=lambda x: rbydaddr(x) if not x.startswith('%') else x,
             action=AppendMrootOrPath,
             help="Block address of the mroots. Defaults to 0x{0,1}.")
     parser.add_argument(
             'paths',
             nargs='*',
-            type=lambda x: rbydaddr(x) if not x.startswith('/') else x,
+            type=lambda x: rbydaddr(x) if not x.startswith('%') else x,
             action=AppendMrootOrPath,
-            help="Paths to show, must start with a leading slash. Defaults "
+            help="Paths to show, must start with %% where %% indicates the "
-                "to the root directory.")
+                "root littlefs directory. Defaults to the root littlefs "
+                "directory.")
     parser.add_argument(
             '--trunk',
             type=lambda x: int(x, 0),