configure: allow the selection of alternate config in the build

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>