uboot-imx

x86: dfi-bt700: Select SERIAL_RX_BUFFER via Kconfig

To support more input characters (longer stings pasted into the U-Boot
prompt) without dropping, lets selects the recently added UART RX
buffer for these boards.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>