lowpan-tools: configure python files installation path

  (original commit: f855787fb3e7fac8bbd2f3d23d1ea6884e776c66)

configure python files installation path or else it will use the
default value /usr/lib/python*, which is wrong on 64bit and multilibs
enabled system

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>