dtc: Don't always generate __symbols__ for plugins

At the moment we generate a __symbols__ node if -@ is specified OR if the
dts has the /plugin/ tag.  That difference in behaviour from handling base
trees is unnecessary and slightly confusing.  It also means it's impossible
to create a plugin without symbols.  Since symbols in a plugin are only
useful in the case of stacked plugins - and libfdt doesn't even support
merging plugin symbols as part of overlay application yet - that's a thing
that might be useful.

So make __symbols__ generation depend only on -@.  We also remove remove
the testcases that checked explicitly for this not very useful behaviour.
Instead we don't use -@ for our basic overlay testcase, and check that
symbols are not generated.

At some point in the future we should add support for symbol merging to
libfdt and add testcases for stacked overlay application.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>