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

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>