configs: rename Zynq defconfig files

These three are Zynq boards and the build procedure is almost the
same.  Having the SoC name prefix "zynq_" would be more consistent.
Also, this is the way in which the Linux Device Trees and the U-Boot
configuration files do.

This commit renames as follows:

   zedboard_defconfig     -> zynq_zed_defconfig
   microzed_defconfig     -> zynq_microzed_defconfig
   xilinx_zc706_defconfig -> zynq_zc706_defconfig

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/avnet/microzed/readme.txt

@@ -7,7 +7,7 @@ available from http://www.zedboard.org .
 
 Steps to create a working system for Microzed:
 
-1) make microzed_defconfig
+1) make zynq_microzed_defconfig
 2) make
 3) copy files BOOT.BIN, u-boot-dtb.img, rootfs.cpio.uboot,
 	uImage, zynq-zed.dtb into your SD card

board/avnet/zedboard/readme.txt

@@ -7,7 +7,7 @@ available from http://www.zedboard.org .
 
 Steps to create a working system for Zedboard:
 
-1) make zedboard_defconfig
+1) make zynq_zed_defconfig
 2) make
 3) copy files BOOT.BIN, u-boot-dtb.img, rootfs.cpio.uboot,
 	uImage, zynq-zed.dtb into your SD card

board/xilinx/zc706/readme.txt

@@ -16,7 +16,7 @@ boot.bin file.
 If you already have the Xilinx tools installed, the following sequence
 will unpack, patch and build the rfs, kernel, uboot, and uboot-spl.
 
-make xilinx_zc706_defconfig
+make zynq_zc706_defconfig
 make uboot-patch
 cp ${XILINX_SDK_LIB}/hwplatform_templates/ZC706_hw_platform/ps7_init.{c,h} \
    output/build/uboot-xilinx-v2014.1/board/xilinx/zynq/