Closes #20

An undocumented feature of uci:get_first() is to cast the returned value
to the same type as the default argument. Thus, supplying a default
value ot type number makes string.format superfluous.

As we don't have any older gluon versions so far, and hope to be able to do all
upgrades with invariant scripts, remove the code for now.

If we ever see the need to add such upgrade scripts, we can just re-add this.

gluon-config-mode: Corrected grammar

gluon-core: force kernel panic on out of memory

This also adds /lib/gluon/functions/sysctl.sh for setting sysctl options.

Various changes and moving stuff around.

This will free about 5 to 12 MB of RAM even on lightly used devices
yielding plenty of RAM for autoupdate to succeed.

Configmode should be exited by using the wizard instead.

In combination with kernel.panic the node will reboot if out of memory.