python: Enforce absolute imports for Python 3 compatibility

Change-Id: Ia88d7fd472f7aed9b97df81468211384981bf6c6
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15983
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

ext: DT autogeneration - Add PyFtd to m5 space

This patch adds pyfdt.py to the m5.ext module. This is used in
succeeding patches for generating and editing dtb files and flat
device trees for DT autogeneration.

The file is in the m5_root/src/python/m5/ext directory, as opposed to
the m5_root/ext, because this library is part of the m5 object space
and linking to the m5_root/ext directory from the SConscript file
in src/python can not be done reliably. Linking from the root level
SConscript is also not an option, because it doesn't have the PySource
method defined.

Cloned from: https://github.com/superna9999/pyfdt
Commit: accbcd254584c9295a18878d32999d0c7c156f8e
Version: 0.3

Change-Id: I928bdc912a9507d1f8a3290acf445c7cae496552
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5961
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>