board/zynqmp: update custom board documentation

Update the zynqmp readme.txt file to use BR2_TARGET_UBOOT_ZYNQMP_PSU_INIT_FILE
option for configuring custom zynqmp boards with Buildroot.  There is no need
for any custom / manual steps since the support is included with Buildroot.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/zynqmp/readme.txt

@@ -86,25 +86,19 @@ updated for any programmable logic or DDR customizations which impact
 psu_init (clock/pin setup & mapping/AXI bridge setup/etc). See
 board/xilinx/zynqmp/ directory of U-Boot for natively supported psu_init
 files. If the psu_init files for your board are not found in U-Boot,
-you need to add them by yourself.
+you need to add them using BR2_TARGET_UBOOT_ZYNQMP_PSU_INIT_FILE.
 
 1) Start with a defconfig supported by Buildroot (e.g. ZCU106)
     make zynqmp_zcu106_defconfig
 
-2) make uboot-menuconfig
+2) make menuconfig
-    Visit the following menu and enable CONFIG_XILINX_PS_INIT_FILE
+    Visit the following menu to configure BR2_TARGET_UBOOT_ZYNQMP_PSU_INIT_FILE
 
-    ARM architecture  --->
+    Bootloaders  --->
-       [*] Zynq/ZynqMP PS init file(s) location
+       U-Boot  --->
+          (<Path to psu_init_gpl.c>) Custom psu_init_gpl file
 
-3) Copy psu_init_gpl.c/h generated by the Xilinx Vivado tools into
+3) make
-   output/build/uboot-custom/board/xilinx/zynqmp/custom_hw_platform/
-
-4) make
-
-Note: The files in step 3 will need to be re-copied after cleaning the
-output directory and at this time, there is no way to save them as
-part of the buildroot configuration, except as a U-Boot patch.
 
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 Important Note