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/ge/mx53ppd/Kconfig b/board/ge/mx53ppd/Kconfig
index bebb2fa..728e9a9 100644
--- a/board/ge/mx53ppd/Kconfig
+++ b/board/ge/mx53ppd/Kconfig
@@ -1,4 +1,3 @@
-
 if TARGET_MX53PPD
 
 config SYS_BOARD
@@ -13,6 +12,9 @@
 config SYS_CONFIG_NAME
 	default "mx53ppd"
 
+config IMX_CONFIG
+	default "board/ge/mx53ppd/imximage.cfg"
+
 source "board/ge/common/Kconfig"
 
 endif