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

This image combines the SPL with the i.MX header, the FCB and U-Boot.

For i.MX25/35/51, the FCB is ignored by the boot ROM, so this image is just
useful because it can be programmed on a NAND Flash page boundary.

For i.MX53, the FCB is required by the boot ROM.

This does not support i.MX6 so far because its FCB is more complicated.

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