rbpaths: Add new special dir HOME_DIR for RaaA.
HOME_DIR is intended for not-so-advanced files which shall be user
visible, and thus not in /.rockbox. Therefore HOME_DIR is translated
to $HOME on RaaA, /sdcard on android, the internal memory on ypr0
and "/" on native targets.
ROCKBOX_DIR ("/.rockbox") already existed as special and is translated
to whatever the real rockbox dir is on the target (e.g. /sdcard/rockbox
on android), but it's not suitable for some files we generate
(e.g. battery-bench.txt).
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@31430 a1c6a512-1295-4272-9138-f99709370657