tools/pyboard.py: Fix joining of path in filesystem_command.

This was broken by 5327cd1 Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

tools/pyboard.py

@@ -671,7 +671,7 @@ def fname_cp_dest(src, dest):
  if dest is None or dest == "":
  dest = src
  elif dest == ".":
- dest = "/".join(".", src)
+ dest = "./" + src
  elif dest.endswith("/"):
  dest += src
  return dest