Default to the alphabetically smallest branch if none is set in
site.conf.

Also prevent enabling the autoupdater when no branches exist.

The new options are CONFIG_GLUON_AUTOUPDATER_BRANCH and
CONFIG_GLUON_AUTOUPDATER_ENABLED and allow to control the default branch
and default enable status separately.

The `or ''` fallback in targets/generic is removed, as GLUON_ENV will
set all variables in GLUON_VARS, making previously non-existing
variables exist with an empty value.

Fixes #1310

Some files have received some additional refactoring.

Fixes #1043

We also make use of the boolean support of simple-uci to make scripts
clearer.

Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

The probability field is removed, updates are performed once per night between
04:00 and 05:00.