imx: mx6ul_14x14_evk discard MX6UL from CONFIG_SYS_EXTRA_OPTIONS

Discard MX6UL from CONFIG_SYS_EXTRA_OPTIONS, since we default
select MX6UL for mx6ul_14x14_evk board.
Signed-off-by: NPeng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>

configs/mx6ul_14x14_evk_defconfig

@@ -4,5 +4,5 @@ CONFIG_CMD_NET=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_DHCP=y
 CONFIG_SPL=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6UL"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg"
 CONFIG_TARGET_MX6UL_14X14_EVK=y