libfdt: Tidy up pylibfdt build rule

libfdt: Tidy up pylibfdt build rule

At present the build rule for pylibfdt depends on _libfdt.so but modern
Python versions add a different suffix to the output file, resulting in
something like _libfdt.cpython-38-x86_64-linux-gnu.so

The result is that pylibfdt is rebuilt every time.

Rename the file the standard name so that the rule works correctly. Also
add libfdt.py to the dependencies, so that file is always created if
missing.

Signed-off-by: Simon Glass <sjg@chromium.org>