zynq: unify readme.txt for all Zynq boards

The readme.txt for the three boards are almost the same, so merge
them into board/zynq/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>

board/xilinx/zc706/readme.txt

@@ -1,29 +0,0 @@
-This is the Buildroot board support for the Xilinx ZC706. The ZC706 is
-a development board based on the Xilinx Zynq-7000 based
-All-Programmable System-On-Chip.
-
-ZC706 information including schematics, reference designs, and manuals
-are available from
-http://www.xilinx.com/products/boards-and-kits/ek-z7-zc706-g.html.
-
-Steps to create a working system for ZC706 board:
-
-1) make zynq_zc706_defconfig
-2) make
-3) All needed files will be available in the output/images directory.
-   Create a FAT32 partition at the beginning of your SD Card and copy files:
-     - boot.bin
-     - u-boot.img
-     - uImage
-     - uramdisk.image.gz (should be renamed from rootfs.cpio.uboot)
-     - devicetree.dtb (should be renamed from zynq-zc706.dtb)
-   into your SD card
-4) boot your ZC706 board
-
-You can alter the booting procedure by creating a file uEnv.txt
-in the root of the SD card. It is a plain text file in format
-<key>=<value> one per line:
-
-kernel_image=myimage
-modeboot=myboot
-myboot=...