scripts: ensure intended path for edtlib imports

When updating `sys.path` to allow importing the pickled edtlib instance,
add the path to the front of `sys.path`, not the end. This ensures that
the `devicetree.edtlib` module that is imported is the one relative
to the files being run, not some other version which may exist on the
path.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>
diff --git a/scripts/dts/gen_defines.py b/scripts/dts/gen_defines.py
index f8d72ed..0f4cc02 100755
--- a/scripts/dts/gen_defines.py
+++ b/scripts/dts/gen_defines.py
@@ -27,8 +27,8 @@
 import re
 import sys
 
-sys.path.append(os.path.join(os.path.dirname(__file__), 'python-devicetree',
-                             'src'))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), 'python-devicetree',
+                                'src'))
 
 from devicetree import edtlib