arm: imx: add u-boot-nand.imx to boot from NAND without SPL

U-Boot can be booted from NAND without SPL by prepending the DCD header to
the actual U-Boot binary. However this requires prepending 1024 bytes to
u-boot.imx (DCD + u-boot.bin).

There is already a similar target to build spl/u-boot-nand-spl.imx, add the
same option for no-SPL boot.

Tested on i.MX6ULL.

The resulting layout of u-boot-nand.imx is:

 - Offset 0x0000 (0 KiB): padding
 - Offset 0x0400 (1 KiB): DCD header
 - Offset 0x1000 (4 KiB): u-boot.bin

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>