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/aristainetos/Kconfig b/board/aristainetos/Kconfig
index cc603c1..2d8fcc5 100644
--- a/board/aristainetos/Kconfig
+++ b/board/aristainetos/Kconfig
@@ -8,6 +8,9 @@
 config SYS_BOARD_VERSION
 	default 5
 
+config IMX_CONFIG
+	default "board/aristainetos/aristainetos2.cfg"
+
 endif
 
 if TARGET_ARISTAINETOS2CCSLB
@@ -20,4 +23,7 @@
 config SYS_BOARD_VERSION
 	default 6
 
+config IMX_CONFIG
+	default "board/aristainetos/aristainetos2.cfg"
+
 endif