It is true that the boot process on ARM is more entangled with the platform then on x86. In contrast to x86 where you have to do additional steps to lock it down on ARM additional work of the system integrator is required to keep it open/customizable. Not sure if we can expect Apple (or anyone else who is not interest in this business case) to invest into that (if they don't explicitly state that the will).

Edit: This touches a lot of areas, like auto-detection and configuration of hardware and sub-components (either connected or embedded into the SoC) or security specific areas like ARM TrustZone which has vendor specific implementations and secure boot procedures