mx6: Select MX6QDL option via Kconfig

Currently the MX6QDL option is selected via CONFIG_SYS_EXTRA_OPTIONS,
but it is better to select it directly via Kconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/configs/mccmon6_nor_defconfig b/configs/mccmon6_nor_defconfig
index 0f9794c..7c46579 100644
--- a/configs/mccmon6_nor_defconfig
+++ b/configs/mccmon6_nor_defconfig
@@ -5,7 +5,7 @@
 CONFIG_TARGET_MCCMON6=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 # CONFIG_CMD_BMODE is not set
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/liebherr/mccmon6/mon6_imximage_nor.cfg,MX6QDL"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/liebherr/mccmon6/mon6_imximage_nor.cfg"
 CONFIG_SPL=y
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_ENV_SUPPORT=y