imx: mx6sx: move MX6SX to Kconfig entry

If including MX6SX in CONFIG_SYS_EXTRA_OPTIONS, CONFIG_ROM_UNIFIED_SECTIONS
will not effect.So move MX6SX to Kconfig entry from CONFIG_SYS_EXTRA_OPTIONS to
"select MX6SX" to boards using i.MX6 SoloX.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/configs/mx6sxsabresd_defconfig b/configs/mx6sxsabresd_defconfig
index f4e88f2..2ea5c89 100644
--- a/configs/mx6sxsabresd_defconfig
+++ b/configs/mx6sxsabresd_defconfig
@@ -1,7 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_MX6SXSABRESD=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sxsabresd/imximage.cfg,MX6SX"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sxsabresd/imximage.cfg"
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y