Update App From SD card

[spanky]

I have an app that is relatively large (too large for the factory + 2 OTA partitions)

My board has an SD card which could easily contain an update image. I'm wondering if the following is possible:

• Build a small image that runs on every cold boot, checks the SD card for a firmware file.
  • If found, write that to the next partition. Then run the next partition
  • If no image file is found on the SD card, just continue on to execute the next partition

This way I would remove the need to maintain and test the OTA code in the main app (since the small "boot" app would not change and also would be relatively simple)
It would also provide a means for recovery (albeit a physical/separate transfer of the firmware image to the SD card)

This is also distinctly different from the documented OTA process since that tags the boot partition once verified. I want to boot from this small helper/updater app, and then immediately run the next app in the next partition. Any suggestions?