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>