Not all firmware allow you to turn off secureboot or enroll your own keys. You’ll see plenty of this on bios-mods.com if you want to know what that looks like. It also really throws into sharp relief questions around things like device ownership.
Let me tell you about my experience with an Amazon DeepLens device (x86-64) that I’ve been trying to get stock Ubuntu installed on. The only keys on this device are Amazon ones. This means I cannot install any OS other than the one they supply (a modified Ubuntu 16.04 install). If I own the device, shouldn’t I be free to install my own OS? If I own the device, and have physical control of it, I should be able to bypass secureboot, period - but not always the case today.