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/freescale/imx8mp_evk/Kconfig b/board/freescale/imx8mp_evk/Kconfig
index 49bb29a..42625fd 100644
--- a/board/freescale/imx8mp_evk/Kconfig
+++ b/board/freescale/imx8mp_evk/Kconfig
@@ -9,6 +9,9 @@
 config SYS_CONFIG_NAME
 	default "imx8mp_evk"
 
+config IMX_CONFIG
+	default "board/freescale/imx8mp_evk/imximage-8mp-lpddr4.cfg"
+
 source "board/freescale/common/Kconfig"
 
 endif