Coreboot has some pretty solid ARM support too. Modern Chromebooks (both x86 and ARM) use Coreboot with a "depthcharge" payload. You can even use Libreboot at this point to bring up an RK3288, and boot into ChromeOS or a traditional linux. Even cooler is that if you do your own build of Coreboot, you can provide your own OS verification keys, and use vbutil to sign your own OS and have custom verified boot.