imx: add container target

To support SPL loading container file, add a new Makefile target,
and introduce a new Kconfig file to source the cfg file which
will be parsed by mkimage.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/imx8/Kconfig b/arch/arm/mach-imx/imx8/Kconfig
index e9496cb..d17760e 100644
--- a/arch/arm/mach-imx/imx8/Kconfig
+++ b/arch/arm/mach-imx/imx8/Kconfig
@@ -29,6 +29,13 @@
 	help
 	  This is to let SPL could load i.MX8 Container image
 
+config IMX_CONTAINER_CFG
+	string "i.MX Container config file"
+	depends on SPL
+	help
+	  This is to specific the cfg file for generating container
+	  image which will be loaded by SPL.
+
 choice
 	prompt "i.MX8 board select"
 	optional