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Microsoft’s Secure Boot has been broken for a decade and no one noticed until now
(www.welivesecurity.com)
This is a most excellent place for technology news and articles.
The difference is that with ARM TrustZone, there is an efuse burned with the key that the manufacturer set in the SoC itself that checks the signature of the primary bootloader, which cannot be modified.
Standard computers do not have such a hardware-level key, so if you wanted to replace the bootloader with something like coreboot if it has been ported to your board, then you can. On smartphones, you do not have that option.
Same thing goes for even more locked down systems like game consoles.