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/beacon/imx8mn/Kconfig b/board/beacon/imx8mn/Kconfig
index 2bcfb25..dceb3de 100644
--- a/board/beacon/imx8mn/Kconfig
+++ b/board/beacon/imx8mn/Kconfig
@@ -17,6 +17,9 @@
 	bool "Enable 2GB LPDDR"
 	default n
 
+config IMX_CONFIG
+	default "arch/arm/mach-imx/imx8m/imximage-8mn-lpddr4.cfg"
+
 source "board/freescale/common/Kconfig"
 
 endif