and the option to do raw read/write to the embedded flash
To use PC bootloader terminology, that's chainloading. The SoC still starts from internal storage first, then switches to SD. My comment was about making a phone without internal storage at all, i.e. SoC boots directly from SD.
All ARM SoCs to my knowledge know enough about their peripherals to get a bootloader off an SD card, so the BIOS/first-user-replacable-code-to-run could easily come from there instead of the storage that's soldered to the motherboard.
To use PC bootloader terminology, that's chainloading. The SoC still starts from internal storage first, then switches to SD. My comment was about making a phone without internal storage at all, i.e. SoC boots directly from SD.