On this page, you can learn how to install a boot-debuggable image to get root access during startup time within the ADB developer tools.
Back to the downloads pagefastboot devices
It should list your device. If it does not detect your device, make sure you have installed the appropriate "ADB drivers" for your device (Google should be your friend to get these)
fastboot oem unlock
Type the following command with the correct file name in the command prompt or terminal:
fastboot flash boot <NAME_OF_THE_BOOT_DEBUGGABLE_IMAGE>.img
To re-lock the bootloader, type the following command in the command prompt or terminal:
fastboot oem lock
To reboot your phone, type the following command in the command prompt or terminal:
fastboot reboot