meson: Alias CONFIG_SOFTMMU -> CONFIG_SYSTEM_ONLY

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>

meson.build

@@ -2864,6 +2864,7 @@ foreach target : target_dirs
     endif
     config_target += { 'CONFIG_BSD_USER': 'y' }
   elif target.endswith('softmmu')
+    config_target += { 'CONFIG_SYSTEM_ONLY': 'y' }
     config_target += { 'CONFIG_SOFTMMU': 'y' }
   endif
   if target.endswith('-user')