Remove obsolete distro variables

We don't use sysvinit anymore, and we should not install task-core-boot
(it makes no difference because it is appended to the RDEPENDS of packagegroup-base)

https://bugzilla.gnome.org/show_bug.cgi?id=690959
Generate only UTF8 layout

It's 2013, people should use UTF8. And it reduces confusion in the Region &
Language panel, because they're not duplicated.

https://bugzilla.gnome.org/show_bug.cgi?id=690959
Remove netbase

We use NetworkManager instead

https://bugzilla.gnome.org/show_bug.cgi?id=690959
Set DISTRO_VARS explicitly instead of appending to the default

This way we get rid of the default x11 distro-feature, which causes
valgrind to depend on libx11.

https://bugzilla.gnome.org/show_bug.cgi?id=690959
Remove .la files from generated images

.la files are unnecessary and dangerous, especially when they reference an
older version of the library (as is the case of glib, which is built
by both yocto and ostbuild)

https://bugzilla.gnome.org/show_bug.cgi?id=690959