LWIP_WND_SCALE macro

LWIP_WND_SCALE and TCP_RCV_SCALE: Set LWIP_WND_SCALE to 1 to enable window scaling. Set TCP_RCV_SCALE to the desired scaling factor (shift count in the range of [0..14]). When LWIP_WND_SCALE is enabled but TCP_RCV_SCALE is 0, we can use a large send window while having a small receive window only.