[aesl] Use the constant CURRENT_LIMIT to limit motor current

This effectively increase the current limit from 400 to 600

While here, make sure the 'current bumper' is really triggered by adding a small margin
[aesl] Improved LEDs handling

- dim the LEDs on the corner to compensate for the absence of wood panels
- make sure RGB values in [0, 255] are converted to [0, 127] (since it seems to be what
  the aseba low-level expect)