qemu: Determinism fixes

When sources are included within debug information, a couple of areas of the
qemu build are not reproducible due to either full buildpaths or timestamps.

Replace the full paths with relative ones. I couldn't figure out how to get
meson to pass relative paths but we can fix that in the script.

Upstream-Status: Pending [some version of all/part of this may be accepted]
RP 2021/3/1

Author: rpurdie

scripts/decodetree.py

@@ -1558,7 +1558,7 @@ def main():
     toppat = ExcMultiPattern(0)
 
     for filename in args:
-        input_file = filename
+        input_file = os.path.relpath(filename)
         f = open(filename, 'rt', encoding='utf-8')
         parse_file(f, toppat)
         f.close()