Previously, the various install paths were obtained using get_option
as needed.
This patch unifies the directory selection inside the top-level meson
file as requested in https://github.com/hughsie/colord/pull/62.
In this mode, both the metadata and firmware is stored on the local filesystem
and distributed using a distribution system like OSTree.
Fixes https://github.com/hughsie/fwupd/issues/162
