boot_serial: Add optional img mgmt slot info feature

[nordicjm]

Adds a minimal version of the slot info feature to serial recovery,
and enables it by default. This feature adds an additional 688 bytes
of flash usage and 16 bytes of RAM usage on a cortex M33 device

[de-nordic]

I am not sure we should bring it by default from the beginning; this increases flash and and surprised developers will be forced to check what happened and disable the feature.

[nordicjm]

What about default if updateable images > 1? The problem that always comes up is (with multi image devices) knowing what "slot" to use to upload to, on single image devices yes this feature is mostly useless (maybe max image size is useful) but for multi image devices especially those that support uploading via MCUboot and application, it gives clarity over where to upload

[nordicjm]

Have moved it to be > 1 image. Though had to add a fix for the app size not being populated (it is populated if you enter serial recovery by way of there being no application which I was testing before, but not through button press). Unfortunately because you now need to duplicate the sector data, it has significantly bumped RAM.
Without:
With:
+724 bytes flash, +4112 bytes RAM. Will ask David if we can just make it static in the file or add a define for it because that is pretty ridiculous
Have found a workaround, back to normal:
Without:

Memory region         Used Size  Region Size  %age Used
           FLASH:       43188 B        64 KB     65.90%
             RAM:       28448 B       448 KB      6.20%

With:

Memory region         Used Size  Region Size  %age Used
           FLASH:       43912 B        64 KB     67.00%
             RAM:       28464 B       448 KB      6.20%

+724 byte flash, +16 bytes RAM