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/ronetix/imx8mq-cm/Kconfig b/board/ronetix/imx8mq-cm/Kconfig
index 9dd6a86..f0a240c 100644
--- a/board/ronetix/imx8mq-cm/Kconfig
+++ b/board/ronetix/imx8mq-cm/Kconfig
@@ -9,4 +9,7 @@
 config SYS_CONFIG_NAME
 	default "imx8mq_cm"
 
+config IMX_CONFIG
+	default "board/ronetix/imx8mq-cm/imximage-8mq-lpddr4.cfg"
+
 endif