Define communication between bootloader and booted image

[d3zd3z]

In order for the application to know how to perform an upgrade, it needs to know certain information from the bootloader, at a minimum, a version number. It would be better, however, for this information to be closer to the information needed to convey (upgrade type, slot information, etc).

Implement a mechanism to communicate this information between the bootloader and the running app.

[d3zd3z]

Copied from MCUB-17.

[nvlsianpu]

Zephyr: Inter-applications flash_map exchange mechanism

[d3zd3z]

I've requested Zephyr: Inter-applications flash_map exchange mechanism be closed, and I'm going to close this as well. If someone finds a need for this, feel free to re-open, but with Zephyr, we seem to do OK by conveying this information through device tree, and the current mechanism.