All boards in mach-mxs support multi image, so we no longer have to make
the board type a choice. Let the user compile all boards for a SoC
together. We cannot compile i.MX23 together with i.MX28 though, there
are still many conflicting defines which use the IMX_ namespace which
are really SoC specific.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>