imx: Add u-boot-with-spl.imx make target

This image combines the SPL with the i.MX header and U-Boot. This is a
convenient way of having a single image to program on some boot devices.

The i.MX header has to be added to the SPL before appending U-Boot, so that the
boot ROM loads only the SPL.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>