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.