zynq_zed: bump U-Boot to xilinx-v2016.2

zynq_zed: bump U-Boot to xilinx-v2016.2

This is the latest release tag in the Xilinx repository, based on
U-Boot v2016.01 in the mainline.

Commit 46d6a561be41 ("zedboard: Generate BOOT.BIN automatically")
switched from the Xilinx GitHub repository to the U-Boot mainline,
but this commit gets it back to the Xilinx GitHub repository for the
following reasons:

 - This defconfig still relies on the Xilinx's local repository for
   the kernel.  The vendor does the best test for the combination of
   U-Boot and the kernel with the same release tag (xilinx-v2016.2
   in this case).

 - At the time of commit 46d6a561be41, the u-boot-xlnx still needed
   manual copy of ps7_init(_gpl).c/h in order to build a working SPL
   image.  So, the mainline U-Boot had advantage in that point of
   time.  However, the improvement in the mainline U-Boot was merged
   into the u-boot-xlnx at the xilinx-v2015.3 release.  Now, the
   mainline and the u-boot-xlnx are even in this point of view.

 - The mainline U-Boot defaults to boot FIT, so something must be
   done; either patch environments around with a local patch to
   switch to uImage booting (current solution), or build an FIT
   with a post build script (chromebook snow does this).  On the
   other hand, the Xilinx repository defaults to uImage booting,
   so it is straightforward, and needs no addition care.

This commit does:

 - Switch to the Xilinx custom git repository, and stick to the
   xilinx-v2016.2 tag.

 - Delete the local patch board/avnet/zedboard/uboot/0001...
   since the Xilinx custom repository can boot uImage by default.

 - Enable BR2_TARGET_UBOOT_FORMAT_IMG instead of ..._FORMAT_DTB_IMG
   since all the Zynq boards in U-Boot enable CONFIG_OF_EMBED.

 - Replace BR2_TARGET_UBOOT_ZYNQ_IMAGE with BR2_TARGET_UBOOT_SPL_NAME
   since U-Boot can natively generate the Zynq boot image now. The
   Zynq image support for mkimage tool was upstreamed at v2016.01
   (so xilinx-v2016.1 as well), so no additional tool is needed
   any more.

 - Update readme.txt

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>