This allows us to easily build just libfwupd in a flatpak manifest without
installing dozens of deps to build things we're just going to delete anyway.
This means we can avoid loading a ton of non-fwupd files, and reduces our
running RSS from 5.4Mb to 2.8Mb. Old versions of appstream-glib caches a lot of
the localization string data which we just don't care about for firmware files.
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
