mpremote/mount: 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

@@ -753,7 +753,12 @@ def ioctl(self, request, arg):
             machine.mem32[arg] = self.seek(machine.mem32[arg], machine.mem32[arg + 4])
         elif request == 4:  # CLOSE
             self.close()
-        return 0
+        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 suported ports. n + 7 should be muliple of 16 to efficiently use
+            # gc blocks in mp_reader_vfs_t.
+            return 249
+        return -1
 
     def flush(self):
         pass