imx: mx7d: move MX7D to Kconfig entry

If including MX7D in CONFIG_SYS_EXTRA_OPTIONS, CONFIG_ROM_UNIFIED_SECTIONS
 will not effect.So move MX7D to Kconfig entry from CONFIG_SYS_EXTRA_OPTIONS to
"select MX7D" to boards using i.MX7 Dual.

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>

arch/arm/cpu/armv7/mx7/Kconfig

@@ -15,6 +15,7 @@ choice
 
 config TARGET_MX7DSABRESD
 	bool "mx7dsabresd"
+	select MX7D
 	select DM
 	select DM_THERMAL
 

configs/mx7dsabresd_defconfig

@@ -3,7 +3,7 @@ CONFIG_ARCH_MX7=y
 CONFIG_TARGET_MX7DSABRESD=y
 CONFIG_IMX_RDC=y
 CONFIG_IMX_BOOTAUX=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx7dsabresd/imximage.cfg,MX7D"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx7dsabresd/imximage.cfg"
 # CONFIG_CMD_BOOTD is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set