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>

board/dfi/dfi-bt700/Kconfig

@@ -25,6 +25,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select SPI_FLASH_STMICRO
 	imply SPI_FLASH_SPANSION
 	imply SPI_FLASH_WINBOND
+	select SERIAL_RX_BUFFER
 
 config PCIE_ECAM_BASE
 	default 0xe0000000