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/warp7/Kconfig b/board/warp7/Kconfig
index c089bca..e5051cd 100644
--- a/board/warp7/Kconfig
+++ b/board/warp7/Kconfig
@@ -20,4 +20,7 @@
 	help
 	  The address the FDT file should be loaded to.
 
+config IMX_CONFIG
+	default "board/warp7/imximage.cfg"
+
 endif