scripts/mtest2make.py: export all-%s-targets variable and use it

There are some places where the conditional makefile support is the
simplest solution. Now we don't expose CONFIG_TCG as a variable create
a new one that can be checked for the check-help output.

As check-tcg is a PHONY target we re-use check-softfloat to gate that
as well.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210202134001.25738-13-alex.bennee@linaro.org>

scripts/mtest2make.py

@@ -110,6 +110,7 @@ def emit_suite(name, suite, prefix):
     print('ifneq ($(filter %s %s, $(MAKECMDGOALS)),)' % (target, prefix))
     print('.tests += $(.test.$(SPEED).%s)' % (target, ))
     print('endif')
+    print('all-%s-targets += %s' % (prefix, target))
 
 targets = {t['id']: [os.path.relpath(f) for f in t['filename']]
            for t in introspect['targets']}