We currently use strings for passing arches and image types around,
which is not ideal. We should have a finite set of supported image types
and architectures as well as upload targets. This PR introduces custom
types to make the code base more readable and possibly also more
correct.
I considered some alternatives like a struct with private fields, but struct cannot
be const, so that does not help either. Eventually I think this is the "get s**t done"
solution.
The package also includes unit-tests which try to convert string to
structure and the other way around to make sure it all works properly.
8dac72f4fd