luci-mod-system: implement system.description, system.notes

Implement two new text "options" for UCI system config, intended to
help humans describe the device.

"system.description" is a short, single-line description suitable for
selector UIs in remote administration applications, or remote UCI (over
ubus RPC), etc.  It would also be suitable as a default for LLDP/SNMP
"system description".

"system.notes" is a multi-line, free-form text field that can be used in
any way the user wishes, e.g. to hold installation notes, or unit serial
number and inventory number, location, etc.

Signed-off-by: Henrique de Moraes Holschuh <henrique@nic.br>