Manual: Platform files - split out the button and action commands from the platform...

Manual: Platform files - split out the button and action commands from the platform files into separate keymap files. This avoids code duplication for targets which share the same keymap (M5,X5; the Archos Recorders and Ondios etc.). Before, someone would have to change a few files if e.g. something changed for the Ipod 4G pad, now only the keymap-ipod4g.tex has to be edited. The shared keymap files contain all possibly needed commands, for instance the radio screen actions in keymap-mx5.tex which is only used for the X5 not the M5.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19262 a1c6a512-1295-4272-9138-f99709370657