__init__: change import of timestr convert methods

Change the import path of the robot internal convert methods.

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>
__init__: fix default value for sdr source

The Sdr source can be selected with SDR or sensor device. That means wheather
use of SDR commands or sensor device command to fetch the SDR data.
The default value is now "sensor device".

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>