Hexpansion EEPROM update mechanism

[Jonty]

Inevitably we are going to end up with code on a hexpansion eeprom that is no longer api-compatible, or that needs updating for other reasons.

We should have a standardised hexpansion patching/auto-update mechanism. We could perhaps also allow apps to hook their associated hexpansions and allow their own code to run instead of the code on the hexpansion.