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/Makefile b/Makefile
index 54da5cd..733bcec 100644
--- a/Makefile
+++ b/Makefile
@@ -1365,9 +1365,17 @@
 	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@
 
 ifeq ($(CONFIG_ARCH_IMX8M)$(CONFIG_ARCH_IMX8), y)
+ifeq ($(CONFIG_SPL_LOAD_IMX_CONTAINER), y)
+u-boot.cnt: u-boot.bin FORCE
+	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@
+
+flash.bin: spl/u-boot-spl.bin u-boot.cnt FORCE
+	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@
+else
 flash.bin: spl/u-boot-spl.bin u-boot.itb FORCE
 	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@
 endif
+endif
 
 u-boot-with-spl.imx u-boot-with-nand-spl.imx: SPL u-boot.bin FORCE
 	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@