tools/mpremote: Add ioctl to specify large read buffer size.

Speeds up importing files from mounted filesystem.

Also fix the return code for invalid / unsupported ioctl requests.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

tools/mpremote/mpremote/transport_serial.py

@@ -753,6 +753,13 @@ class RemoteFile(io.IOBase):
             machine.mem32[arg] = self.seek(machine.mem32[arg], machine.mem32[arg + 4])
         elif request == 4:  # CLOSE
             self.close()
+        elif request == 11:  # BUFFER_SIZE
+            # This is used as the vfs_reader buffer. n + 4 should be less than 255 to
+            # fit in stdin ringbuffer on supported ports. n + 7 should be multiple of 16
+            # to efficiently use gc blocks in mp_reader_vfs_t.
+            return 249
+        else:
+            return -1
         return 0
 
     def flush(self):