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>