Added Miyoo frontend support

[nicjansma]

Adds miyoogamelist.xml frontend support (for Onion OS on Miyoo Mini devices).

miyoogamelist.xml is like gamelist.xml for EmulationStation, but only supports <path>, <name> and <image> for each <game>.

e.g.

<?xml version="1.0"?>
<gameList>
  <game>
    <path>./3-D Tetris (USA).7z</path>
    <name>3-D Tetris (USA)</name>
    <image>./Imgs/3-D Tetris (USA).png</image>
  </game>
...

OnionOS also assumes media (screenshots) will be in an Imgs/ subfolder. No other media is supported, and this change allows a frontend to specify NULL for paths (such as marquees or covers) so they won't get created/written.