Add instruction for password changing

BUILD.md

@@ -1,5 +1,4 @@
-ReadMe for VMG8825-B50B V5.17(ABNY.7)C0
-
+Build guide for VMG8825-B50B V5.17(ABNY.7)C0
 
 0. Introduction
 
@@ -80,3 +79,21 @@ ReadMe for VMG8825-B50B V5.17(ABNY.7)C0
 
     The firmware image will locate at <extrated directory>/bin/brcm963xx/zyxel/ras.bin
     You can use it update VMG8825-B50B by using the firmware update procedure.
+
+7. Change default password
+
+  Make a copy of the sysconfig.tar.gz file located in NOMEFIRMWARE/build_dir/target-arm_v7-a_uClibc-0.9.32_eabi/root-brcm963xx/etc/
+  Extract it with tar xzvf sysconfig.tar.gz
+  Using your preferred editor (vi, nano, leafpad...), modify both files as follows:
+  zcfg_config.json to set, in plain text, a password for each user (example, under "Username": "admin"; fill in "Password": "NEWPASSWORD").
+  zcfg_predef.json change "AdminRandomPassword": true to false
+  
+  Repack: tar czvf sysconfig.tar.gz {zcfg_config.json,zcfg_predef.json}
+  Copy the new sysconfig.tar.gz back to its place
+
+8. Reassemble firmware
+  From the main firmware folder, run the command ```make merge_firmware``` and wait for it to finish. At this point you will have the new firmware (ras.bin) in FIRMWAREFOLDER/bin/brcm963xx/zyxel/
+
+9. Flash the firmware (resetting everything to factory defaults)
+  From the graphical menu, no explanation needed
+  Via bootloader, just search the forum, anyway, ATUR ipaddresstftpserver:releasename