gdk-pixbuf: create loader cache at build time

gdk-pixbuf: create loader cache at build time

Create the gdk pixbuf loaders cache file at build time rather than run
time, since the root filesytem can be read-only.
We also didn't cater for systemd-based systems, so this solves two
issues at once.

The only side-effect is that we must now pull host-gdk-pixbuf into
target gdk-pixbuf dependencies, but generally users will have gdk-pixbuf
selected if they use libgtk2/3 which already pull in host-gdk-pixbuf.

And remove initscript correspondingly.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>