board/zynq: update custom board documentation

The readme.txt file for zynq boards has some outdated information for the
custom board section.  The custom ps7_init_gpl.c is now defined by
CONFIG_XILINX_PS_INIT_FILE and Buildroot always puts the Xilinx u-boot builds
in the output/build/uboot-custom directory.

This patch updates the instructions in the readme.txt file accordingly.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/zynq/readme.txt

@@ -62,7 +62,7 @@ the U-Boot source tree. Without those files, boot.bin will be built
 successfully but it will not be functional at all. Those files are
 output from the Xilinx tools, but for convenience, U-Boot includes the
 default ps7_init_gpl.c/h of popular boards. Those files may need to be
-updated for any programmable logic customizations which impact
+updated for any programmable logic or DDR customizations which impact
 ps7_init (clock/pin setup & mapping/AXI bridge setup/etc). See
 board/xilinx/zynq/ directory of U-Boot for natively supported ps7_init
 files. If the ps7_init files for your board are not found in U-Boot,
@@ -72,13 +72,13 @@ you need to add them by yourself.
     make zynq_zed_defconfig
 
 2) make uboot-menuconfig
-    Visit the following menu and enable CONFIG_ZYNQ_CUSTOM_INIT.
+    Visit the following menu and enable CONFIG_XILINX_PS_INIT_FILE
 
     ARM architecture  --->
-       [*] Use custom ps7_init provided by Xilinx tool
+       [*] Zynq/ZynqMP PS init file(s) location
 
-3) Copy ps7_init_gpl.c/h generated by the Xilinx tools into
-   output/build/uboot-xilinx-<pkg version>/board/xilinx/zynq/custom_hw_platform/
+3) Copy ps7_init_gpl.c/h generated by the Xilinx Vivado tools into
+   output/build/uboot-custom/board/xilinx/zynq/custom_hw_platform/
 
 4) make