Fix the foe code of the rtl_slavedemo application

[bnjmnp]

With d9ebdc9 the FOE part of the API changed a bit, but the application was not updated.

[nakarlsson]

For this particular example we could also add that it will abort with Error if not in BOOTSTRAP state.
Makes the sample more clear if some might wonder why it is missing. Of course, as a static variable it would be 0'ed by crt as part of .bss otherwise.

.write_only_in_boot = 1;

[bnjmnp]

Good point, I just added what you suggested.

[bnjmnp]

Hi there! I was wondering if there are any updates or specific reasons why this hasn't been merged yet. Is there anything I can do to help move it forward?

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