fontcache: Fix build dependency errors

fontcache: Fix build dependency errors

This addresses warnings like:

WARNING: QA Issue: liberation-fonts rdepends on fontconfig-utils but its not a build dependency? [build-deps]

since the dependencies were being added at package time and were not
visible to bitbake.

Also take the opportunity to convert to use PACKAGEFUNCS rather than
the horrible populate_packages_append.

(From OE-Core rev: f784e9f53f333f05b3ef570334bf2f6bc9e972c4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>