iMX28: Add image header generator tool

This tool can now generate proper image for "BootStream" files.

NOTE: This tool now works only for NAND.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>