This is something I did not know. In the mobile world the boot happens in the UEFI mould right? So when someone licenses from ARM they get to design their own boot sequence?
Booting such a system is highly dependent upon the board configuration and the peripherals that are needed. Furthermore, vendors have different boot-up requirements. Some boot loaders perform proprietary system checks before loading the RTOS. Some boot loaders need to be able to upgrade firmware from an image on-chip, in flash, or fed into it over SPI or a UART. Some boot loaders are little more than a jump table decoder that just jumps to the currently active firmware image on the chip. Creating a universal boot loader with enough flexibility for each of these situations is harder than just implementing a boot loader for a particular electronics product or product family.
The Broadcom parts work like this for the VC4; the boot ROM can talk to an SD card, parse a FAT filesystem, and load and run the second-stage loader (bootcode.bin). But Allwinner chips do this too, and they're even smart enough to try several different media types (including SATA, IIRC!). Ditto Tegra parts.
I imagine that if you're a major customer you get to choose the contents of the boot ROM.
From a hacker perspective, it's brilliant, because the devices are completely unbrickable. It doesn't matter how broken they get, you can't change the ROM, which means that you always have ability to get something working. But, of course, they all have entirely different APIs. The VC4 just dumps an image into memory, sets a couple of registers, and jumps to it, and I imagine the others all do the same thing.