configure: allow the selection of alternate config in the build

While the default config works well enough it does end up enabling a
lot of stuff. For more minimal builds we can select a different list
of devices and let Kconfig work out what we want. For example:

  ../../configure --without-default-features \
    --target-list=arm-softmmu,aarch64-softmmu \
    --with-devices-aarch64=minimal

will override the aarch64-softmmu default set of devices with a more
minimal set of devices that just enables the virt and sbsa-ref models.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210707131744.26027-6-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index 71c7462..651c3b1 100644
--- a/meson.build
+++ b/meson.build
@@ -1595,9 +1595,10 @@
                                                configuration: config_target_data)}
 
   if target.endswith('-softmmu')
+    config_input = meson.get_external_property(target, 'default')
     config_devices_mak = target + '-config-devices.mak'
     config_devices_mak = configure_file(
-      input: ['configs/devices' / target / 'default.mak', 'Kconfig'],
+      input: ['configs/devices' / target / config_input + '.mak', 'Kconfig'],
       output: config_devices_mak,
       depfile: config_devices_mak + '.d',
       capture: true,