Adding the fact that GLUON_RELEASE is needed with 'make manifest'
if it is used with 'make'.

This fact was mentioned in the v2017.1 release notes but is not (clearly)
mentioned inside other parts of the documentation.

Signed-off-by: Chrissi^ <chris@tinyhost.de>

[Matthias Schiffer: minor rewording]

Adding hints that the next_node name should contain a dot to avoid (mobile) browsers interpreting "nextnode" as searchphrase. Also setting it to an fqdn helps when clients use static dns servers.

gluon-core: dns-cache documentation polish
Now its clear that upstreams severs normally means "community gateways".
* added cacheentries to description
* made upstream server description more general
* description of the A and AAAA-records for next_node added
* mention of Cache not being allocated when not set in site.conf 
* dns-cache doku polish

propose to make uci commit more specific to minimize unwanted changes in other temporary written config files like wireless ssid (in some offline changers)

Use the common term; the "Expert Mode" never was a distinct mode of
operation anyways.

This new feature introduces the new uci section 'gluon-core.wireless' with a preserve_channels option:
 * preserve_channels (boolean)

By setting this option to 1 (true) wifi channels will be preserved during upgrades.

Fixes #574

fix typo and express sentence regarding update priority clearer

We'll soon have other output besides the images, having a single directory
for all output makes sense.

Fixes #376