Remove obsolete distro variables.

We don't use sysvinit and we shouldn't install task-core-boot.
Generate only UTF8 layout.

People should use UTF8 already.
Remove netbase.

We use connman instead.
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.
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).