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/imx-common/Kconfig b/arch/arm/imx-common/Kconfig
new file mode 100644
index 0000000..37b3752
--- /dev/null
+++ b/arch/arm/imx-common/Kconfig
@@ -0,0 +1,2 @@
+config IMX_CONFIG
+	string