You unlock bootloader by toggling "OEM unlocking" in Developer Options. To access Developer Options, go to settings > About Phone > Build Number (might be under software info) > touch "Build Number" 7 times until it says Developer Options is on/activated. Then go back to Settings and scroll down to Developer Options and toggle on " OEM/bootloader unlocking". Also toggle on USB debugging. Now you can root using any method for a J7.