dts: separate DT libraries from gen_defines.py

We are now in the process of extracting edtlib and dtlib into a
standalone source code library that we intend to share with other
projects.

Links related to the work making this standalone:

    https://pypi.org/project/devicetree/
    https://python-devicetree.readthedocs.io/en/latest/
    https://github.com/zephyrproject-rtos/python-devicetree

This standalone repo includes the same features as what we have in
Zephyr, but in its own 'devicetree' python package with PyPI
integration, etc.

To avoid making this a hard fork, move the code that's being made
standalone around in Zephyr into a new scripts/dts/python-devicetree
subdirectory, and handle the package and sys.path changes in the
various places in the tree that use it.

From now on, it will be possible to update the standalone repository
by just recursively copying scripts/dts/python-devicetree's contents
into it and committing the results.

This is an interim step; do NOT 'pip install devicetree' yet.
The code in the zephyr repository is still the canonical location.

(In the long term, people will get the devicetree package from PyPI
just like they do the 'yaml' package today, but that won't happen for
the foreseeable future.)

This commit is purely intended to avoid a hard fork for the standalone
code, and no functional changes besides the package structure and
location of the code itself are expected.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

scripts/gen_handles.py

@@ -38,6 +38,11 @@ from elftools.elf.elffile import ELFFile
 from elftools.elf.sections import SymbolTableSection
 import elftools.elf.enums
 
+# This is needed to load edt.pickle files.
+sys.path.append(os.path.join(os.path.dirname(__file__),
+                             'dts', 'python-devicetree', 'src'))
+from devicetree import edtlib  # pylint: disable=unused-import
+
 if LooseVersion(elftools.__version__) < LooseVersion('0.24'):
     sys.exit("pyelftools is out of date, need version 0.24 or later")