Overhaul of the 'system options' chapter in the manual, introducing a few new options...

Overhaul of the 'system options' chapter in the manual, introducing a few new options in the platform files. These make it possible to in- or exclude parts easier and more obvious based on the features of a target and follow the 'features.txt' that's used for the language files (just the check for flash based vs. hdd based player would be inverted). The idea was to be able to parse the features.txt into these platform files automatically later on.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@15972 a1c6a512-1295-4272-9138-f99709370657