I just noticed this Android Security Reference. It has a few pages on boot phase:
https://github.com/doridori/Android-Security-Reference
https://github.com/doridori/Android-Security-Reference/blob/master/boot/verified_boot.md
https://github.com/doridori/Android-Security-Reference/blob/master/boot/bootloader.md
https://github.com/doridori/Android-Security-Reference/blob/master/boot/boot_process.md
