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/arch/arm/mach-imx/Kconfig b/arch/arm/mach-imx/Kconfig
index c5923e3..dd4f027 100644
--- a/arch/arm/mach-imx/Kconfig
+++ b/arch/arm/mach-imx/Kconfig
@@ -5,7 +5,9 @@
 	bool
 
 config IMX_CONFIG
-	string
+	string "DCD script to use"
+	depends on MACH_IMX
+	default "arch/arm/mach-imx/spl_sd.cfg"
 
 config ROM_UNIFIED_SECTIONS
 	bool