Backup factory application

[baptiste-cartier]

Hello everyone,

We have been testing and using mcuboot for some time now and we are pretty happy with it.
After some feedback, the need of a default factory application has been validated.

The use of this default factory application is a backup in case both primary and secondary application were detected as invalid, and erased. This application task is to download a new software (via serial, FOTA, ...) in the secondary slot to try and recover the device.

This last line of backup will have its own set of security keys, different that the one used for the classic applications, and will also be stored signed and encrypted in an external Flash. It must never be erase by the bootloader in the external Flash, even if it cannot be validated. This slot should also only be read if and only no valid application can be booted.

This backup slot could be interpreted as a tertiary slot for an image I guess.

As far as I know, something like that was in the making (#207), but it does not seem to be implemented yet.

How would one implement this in mcuboot (ideally without modifying too much of the code base) ? By reading the code I don't believe setting MCUBOOT_IMAGE_NUMBER to 2 instead of 1 is sufficient.

Regards,
BCA