sensor_shell: add CONFIG to support trigger for more than one sensor device

Before, only one sensor device was supported. It
was possible to set a trigger on a second sensor
device, but the filtering of the channel data
would cause no channels from the new sensor
device to be read.

Also in trigger handler log of sampled data, print
the sensor name and channe name (instead of channel
number).

Signed-off-by: Mike J. Chen <mjchen@google.com>