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>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 2145557..c0a0fd8 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -767,6 +767,8 @@
 
 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"