Recent dtc/libfdt can use either Make or meson as the build system.
By using a subproject, our own meson.build can remove the hard
coded list of source files.
This is also the first step towards managing downloads with .wrap
files instead of submodule.
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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