Convert CONFIG_MX7D to Kconfig

This converts the following to Kconfig:
   CONFIG_MX7D

Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/colibri_imx7_defconfig b/configs/colibri_imx7_defconfig
index 70455ec..bfd257d 100644
--- a/configs/colibri_imx7_defconfig
+++ b/configs/colibri_imx7_defconfig
@@ -12,7 +12,7 @@
 CONFIG_IMX_BOOTAUX=y
 CONFIG_IMX_HAB=y
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/toradex/colibri_imx7/imximage.cfg,MX7D"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/toradex/colibri_imx7/imximage.cfg"
 CONFIG_BOOTDELAY=1
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_USE_PREBOOT=y