migration-test: Only generate a single target architecture

Several changes:
- We only allow generate header "inside" the tree.  Why?  Because we
  need to connit the result, so it makes no sense to generate them on
  the build dir.
- We only generate a single target each time.  Getting all the
  cross-compilers correctly is an impossible task.  So know you do:
     make -C tests/migration $target (native)
     make CROSS_PREFIX=foo- -C tests/migratiion $target (cross)
  And you are done.

- If we are building out of tree, we have no data about if we are
  cross-compile or whatever.  So instead of guess what is happening,
  just do what I pointed on previous point.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20180913132313.11370-1-quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

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