Adds mplayer as possible utility for demo sounds
Adds mplayer as possible utility for demo sounds

To play demo notification sounds users need 'omxplayer'
(Raspbian builtin) or 'afplay'  (OS X builtin) installed on their
path.

This commit adds 'mplayer' (available for all major operating
systems) to the list of possible CLI audio players for the demo.

shikhar394/uptane,uptane/uptane,uptane/uptane,awwad/uptane,shikhar394/uptane,awwad/uptane