boot/uboot: add support for i.MX28 NAND format

Allow to generate 'u-boot.nand', Freescale i.MX28 BootStream format (.sb)
with a header for booting from NAND flash.

There are two possibilities when preparing an image writable to NAND flash:
1) The NAND was not written at all yet or the BCB (Boot Control Blocks) is
   broken. In this case, the NAND image 'u-boot.nand' needs to written.
2) The NAND flash was already written with a good BCB. This applies after
   'u-boot.nand' was correctly written. There is no need to write the BCB
   again. In this case, the bootloader can be upgraded by writing 'u-boot.sb'.

To satisfy both cases 'u-boot.nand' as well as the make target 'u-boot.sb' are
copied to the binaries directory.

mxsboot for NAND images needs all three parameters typed in as integer values
(hex values do not work). The default values choosen are typical sizes for a
NAND flash.

For more information see:
http://www.denx-cs.de/doku/?q=m28evkrunuboot

Commit Note: This patch is based on top of
"[PATCH v2 1/2] boot/uboot: add support for i.MX28 SD format"
http://patchwork.ozlabs.org/patch/453116/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>