Much cleaner version of r21411 - add three column button tables capability to the...

Much cleaner version of r21411 - add three column button tables capability to the manual.  To use, add remote_keymap to the platform file, then button tables can be used in the form Main Button & Remote Button & Description. If remote_keymap is defined in the platform file but additional buttons are not added to the button tables, then the decription will appear in the remote key column, but the manual should still build.  Thanks to bluebrother for pointing out!

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@21423 a1c6a512-1295-4272-9138-f99709370657