patman: Tidy up sys.path changes

patman: Tidy up sys.path changes

Now that we are using absolute paths we can remove some of the sys.path
mangling that appears in the tools.

We only need to add the path to 'tools/' so that everything can find
modules relative to that directory.

The special paths for finding pylibfdt remain.

Signed-off-by: Simon Glass <sjg@chromium.org>