create-sys-script: adapt to separate build dir, modernize, add more checks

The script wasn't apparently used since the switch to meson, because
it required the sys subdirectory to be present in the same subdirectory
where the output script is located.

Let's use f-strings to make the whole thing more readable. Add some
extra checks.

test/sys-script.py

@@ -32,7 +32,6 @@ def f(path, mode, contents):
         f.write(contents)
     os.chmod(path, mode)
 
-
 if len(sys.argv) < 2:
     exit("Usage: {} <target dir>".format(sys.argv[0]))