minikconfig: implement allnoconfig and defconfig modes

minikconfig: implement allnoconfig and defconfig modes

Apart from defconfig (which is a no-op),
allyesconfig/allnoconfig/randcondfig can be implemented simply by ignoring
the RHS of assignments and "default" statements.  The RHS is replaced
respectively by "true", "false" or a random value.

However, allyesconfig and randconfig do not quite work, because all the
files for hw/ARCH/Kconfig are sourced and therefore you could end up
enabling some ARM boards in x86 or things like that.  This is left for
future work, but I am leaving it in to help debugging minikconf itself.

allnoconfig mode is tied to a new configure option, --without-default-devices.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>