pylibfdt: use fallback version in tarballs

When building pylibfdt from the released tarballs[1] setup.py will fail
with the following.

> LookupError: setuptools-scm was unable to detect version for dtc.
> Make sure you're either building from a fully intact git repository or
> PyPI tarballs. Most other sources (such as GitHub's tarballs, a git
> checkout without the .git folder) don't contain the necessary metadata
> and will not work.

seutptools_scm supports a 'fallback_version' that will use the provided
string as the version when the git repo is not available.

[1] https://www.kernel.org/pub/software/utils/dtc/dtc-1.7.0.tar.xz

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

setup.py

@@ -22,6 +22,9 @@ srcdir = os.path.dirname(__file__)
 with open(os.path.join(srcdir, "README.md"), "r") as fh:
     long_description = fh.read()
 
+with open(os.path.join(srcdir, "VERSION.txt"), "r") as fh:
+    version = fh.readline().strip()
+
 def get_top_builddir():
     if '--top-builddir' in sys.argv:
         index = sys.argv.index('--top-builddir')
@@ -51,6 +54,8 @@ setup(
     name='libfdt',
     use_scm_version={
         "root": srcdir,
+        "fallback_version": version,
+        "fallback_root": srcdir,
     },
     cmdclass = {'build_py' : build_py},
     setup_requires = ['setuptools_scm'],