osd.py: rename function parameter on functions prepare_disk/prepare

Used name for function parameter redefining name 'activate' function
from outer scope.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd.py: fix import of os/os.path

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>