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/technexion/pico-imx8mq/Kconfig b/board/technexion/pico-imx8mq/Kconfig
index 031fc1d..628b051 100644
--- a/board/technexion/pico-imx8mq/Kconfig
+++ b/board/technexion/pico-imx8mq/Kconfig
@@ -9,4 +9,7 @@
 config SYS_CONFIG_NAME
 	default "pico-imx8mq"
 
+config IMX_CONFIG
+	default "arch/arm/mach-imx/imx8m/imximage.cfg"
+
 endif