OEM unlocking via Fastboot
If you are using the userdebug build, it behaves like a standard Android device and says device is locked. You may need to unlock the device by following below steps:
- Settings => System => About Tablet => Build number
- Keep on tapping until you see a prompt that says "You are now a developer!"
- Settings => System => Advanced => Deveoper options => OEM unlocking
- reboot to bootloader via
$ adb reboot bootloader
or on Serial terminal
# reboot bootloader
- Type "fastboot 0" in the U-Boot command line
- Run "sudo `which fastboot` oem unlock" from the Host PC
- Wait until the unlock process is complete
Now you should have you device OEM unlocked, and from now onwards you can flash your custom built images generated from your Android build.