scripts: run_common: make edtlib importable to all runners

Runners can (and many should) depend on the devicetree to decide what
to do, especially as it relates to the flash layout. Make it so that
they do not have to manipulate sys.path to get a hold of the edtlib
directory by doing it in one place, before importing any of them.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/scripts/west_commands/run_common.py b/scripts/west_commands/run_common.py
index d614c1c..af82787 100644
--- a/scripts/west_commands/run_common.py
+++ b/scripts/west_commands/run_common.py
@@ -22,6 +22,12 @@
 from west.configuration import config
 import yaml
 
+from zephyr_ext_common import ZEPHYR_SCRIPTS
+
+# Runners depend on edtlib. Make sure the copy in the tree is
+# available to them before trying to import any.
+sys.path.append(str(ZEPHYR_SCRIPTS / 'dts'))
+
 from runners import get_runner_cls, ZephyrBinaryRunner, MissingProgram
 from runners.core import RunnerConfig
 import zcmake