Check that the program counter is in our archive

[mkeeter]

Calling something like

humility -a build-rot-carrier-image-b.zip tasks

will "work" even if Hubris is executing out of slot A, as long as slot B is programmed with the given image. This is because the RoT slots are absolutely addressed in flash, so "check if the image ID at flash location X matches" passes.

Obviously, all Humility results will be wrong when operating in this regime.

We should check the program counter and make sure we're actually in the executable space of our archive.