tools: zynqimage: Add Xilinx Zynq boot header generation to mkimage

As with other platforms vendors love to create their own boot header
formats. Xilinx is no different and for the Zynq platform/SoC there
exists the "boot.bin" which is read by the platforms bootrom. This
format is described to a useful extent within the Xilinx Zynq TRM.

This implementation adds support for the 'zynqimage' to mkimage. The
implementation only considers the most common boot header which is
un-encrypted and packed directly after the boot header itself (no
XIP, etc.). However this implementation does take into consideration the
other fields of the header for image dumping use cases (vector table and
register initialization).

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>