1) On all non PLA plugin, power button is used for exit. For consistency,
we set PLA_EXIT to power button.
2) Manual updated accordingly
Change-Id: Ia4b8fce13bad2f8dc0710cdb29ccf6c74b3fd9c4
Reviewed-on: http://gerrit.rockbox.org/91
Reviewed-by: Thomas Martitz <kugel@rockbox.org>
Reviewed-by: Amaury Pouly <amaury.pouly@gmail.com>