boot/uboot: Generate BOOT.BIN file when building for Xilinx Zynq

This commit integrates generation of a bootable image
for Xilinx Zynq. The generation is independent on Xilinx
flow and utilizes the host-zynq-boot-bin package. The only
required step is to generate a proper U-Boot SPL (instead
of the FSBL).

The SPL generation might work when providing the working
ps7_init.c file to U-Boot. However, from U-Boot 2015.07
a set of generic ps7_init.c files is included and used to
build the U-Boot SPL for various boards. The ps7_init.c file
has been released under GNU/GPL license for this purpose.
For details, see

http://lists.denx.de/pipermail/u-boot/2015-April/210664.html



The SPL searchs for u-boot-dtb.img when booting so we
enforce using of the BR2_TARGET_UBOOT_FORMAT_DTB_IMG format.

[Thomas: remove select of BR2_PACKAGE_HOST_ZYNQ_BOOT_BIN, since this
option no longer exists.]

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