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/apalis-imx8/Kconfig b/board/toradex/apalis-imx8/Kconfig
index c680d63..b43d628 100644
--- a/board/toradex/apalis-imx8/Kconfig
+++ b/board/toradex/apalis-imx8/Kconfig
@@ -25,6 +25,9 @@
 config TDX_CFG_BLOCK_OFFSET
 	default "-512"
 
+config IMX_CONFIG
+	default "board/toradex/apalis-imx8/apalis-imx8-imximage.cfg"
+
 source "board/toradex/common/Kconfig"
 
 endif