chromiumos has different needs for fontconfig than is typical. Our list of
fonts is the same for every user (there is only one linux user anyway).
Also, the list can never change.

We modify the fontconfig ebuild to support patching fonts.conf to have only one
directory to search for fonts (/usr/share/fonts). We also output the caches to
/usr/share/cache/fontconfig because /var isn't located on the readonly
partition. In addition we remove the rescan directive.