base.bbclass: detect when S has been set incorrectly
Currently base.bbclass is creating S if it's not created by unpacking
an archive or fetching a repository. If we avoid creating S we can detect
when S hasn't been set correctly, since it will not exist. Then we can tell
the user that they should set S to a proper value, instead of just failing
with odd errors in later tasks.
Besides removing the auto-creation of S this change will introduce a warning
if S is set incorrectly. The reason for not display an error and return
is due to all external layers that might have recipes that will fail otherwise
and that might be a bit to hard to start with. So use a warning until people
have had a chance to cleanup affected recipes.
