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>