We use the CONFIG_USER_ONLY key to describe user emulation,
and the CONFIG_SOFTMMU key to describe system emulation. Alias
it as 'CONFIG_SYSTEM_ONLY' for parity with user emulation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230613133347.82210-8-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>