ARM: iMX: define an IMX_CONFIG Kconfig option

IMX_CONFIG is currently passed via the SYS_EXTRA_OPTIONS which is marked
as deprecated.

Add a new Kconfig file under arch/arm/imx-common and define the
IMX_CONFIG Kconfig in there.

Each board is supposed to provide a default value pointing to the
appropriate imximage.cfg file.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

arch/arm/Kconfig

@@ -767,6 +767,8 @@ source "arch/arm/cpu/armv7/Kconfig"
 
 source "arch/arm/cpu/armv8/Kconfig"
 
+source "arch/arm/imx-common/Kconfig"
+
 source "board/aristainetos/Kconfig"
 source "board/BuR/kwb/Kconfig"
 source "board/BuR/tseries/Kconfig"

arch/arm/imx-common/Kconfig

@@ -0,0 +1,2 @@
+config IMX_CONFIG
+	string