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>