gluon-config-mode-geo-location-osm extends the
gluon-config-mode-geo-location with a location picker based on
OpenStreetMaps.

Based-on-patch-by: Jan-Tarek Butt <tarek@ring0.de>

As already done with other config mode texts, the altitude field now has
default texts that are used when they are not set in the site i18n files.
The altitude-help text has been removed from site i18n; instead, the
geo-location-help text now overrides the whole section description
including the part that mentions the altitude.

Do not show by default, delete old altitude value when a form without
altitude shown is saved.

Fixes #1493

simple-uci will now delete an option when nil is passed.

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