imx: Finish migration of IMX_CONFIG to Kconfig

- Provide a default Kconfig value of the default script
- Largely continue to define this via the board Kconfig file
- For the boards that select a script based on defconfig rather than
  TARGET, keep this within the defconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/toradex/colibri_imx7/Kconfig b/board/toradex/colibri_imx7/Kconfig
index d33ec63..87c416a 100644
--- a/board/toradex/colibri_imx7/Kconfig
+++ b/board/toradex/colibri_imx7/Kconfig
@@ -71,6 +71,9 @@
 
 endif
 
+config IMX_CONFIG
+	default "board/toradex/colibri_imx7/imximage.cfg"
+
 source "board/toradex/common/Kconfig"
 
 endif