POST really doesn't make sense in many embedded systems and most modern devices I've worked on or with don't use them. The general approach to boot has been to just try and boot but have a safe failure path. That is, if DRAM times out/a bus error is raised, fail out to an embedded bootloader which can boot over USB/SWD. If flash doesn't respond, fail out. This is exactly what Apple is doing with DFU. It makes sense too; why waste time explicitly testing when you can just "test" it by booting?

Sure, maybe if you're building something that has life and limb risks, POST it up before coming online, but most consumer electronics have no need.