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>
diff --git a/Makefile b/Makefile
index 0f37c4b..15f48cf 100644
--- a/Makefile
+++ b/Makefile
@@ -1522,6 +1522,9 @@
 u-boot.uim: u-boot.bin FORCE
 	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@
 
+u-boot-nand.imx: u-boot.imx FORCE
+	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@
+
 u-boot-with-spl.imx u-boot-with-nand-spl.imx: SPL $(if $(CONFIG_OF_SEPARATE),u-boot.img,u-boot.uim) FORCE
 	$(Q)$(MAKE) $(build)=arch/arm/mach-imx $@