Reboot the device from fastboot mode into fastboot mode again

Communicate with connected Android devices when in bootloader mode (the one place `adb` doesn't work).
More information: <https://cs.android.com/android/platform/superproject/+/master:system/core/fastboot>.
Trick copyright: tl;dr; <https://github.com/tldr-pages/tldr>

fastboot reboot bootloader

click the source code to copy install fastboot on any operating system with command-not-found.com