imx: Fix missing spl_sd configuration for wandboard.

In commit 6e6cf015e7cdd7ca83a933320a81201972bd5e5e ("Merge
git://www.denx.de/git/u-boot-imx") the line defining spl_sd
configuration for wandboard was removed, which resulted in no SPL
target being built.

Add it back.

Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>

configs/wandboard_defconfig

@@ -12,6 +12,7 @@ CONFIG_CMD_HDMIDETECT=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_BOOTCOMMAND="run findfdt; run finduuid; run distro_bootcmd"
 # CONFIG_CONSOLE_MUX is not set
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 # CONFIG_DISPLAY_BOARDINFO is not set