RaaA: Improve tagcache search to make the database built.

RaaA: Improve tagcache search to make the database built.

First, it add the ability to tagcache to walk through multiple search roots.
Second, it adds symlinks targets to the search roots if they're are not inside any of the current search roots, otherwise the symlink is ignored (unless it's a file).

The default search root is still /, so no search root will be actually added.
But the tagcache now isn't trapped by recursive symlinks anymore and successfully builds, and it's prepared for a future music directory setting.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@28884 a1c6a512-1295-4272-9138-f99709370657