Genex: Add a nullary form for CONFIG

This is consistent with other similar expressions such as PLATFORM_ID,
and makes the CONFIGURATION expression obsolete.

Fix an off-by-one error in
GeneratorExpressionContent::EvaluateParameters exposed by a unit test.

Remove the test for 'bad' nullary use of $<CONFIG>.

Add a unit test to verify that $<CONFIG> and $<CONFIGURATION> have
the same value.
Merge topic 'minor-cleanups' into next

5d3835d Genex: Add a nullary form for CONFIG