Makefile: Restrict system emulation and tools objects

Makefile: Restrict system emulation and tools objects

Restrict all the system emulation and tools objects with a
Makefile IF (CONFIG_SOFTMMU OR CONFIG_TOOLS) check.

Using the same description over and over is not very helpful.
Use it once, just before the if() block.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200118140619.26333-4-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>