CI: add cache for builds

See merge request freifunk-region/stuttgart/site!4

Build cache is kept per build target. Each tag also has a seperate build
cache, so tagged builds are always built from scratch.

CI: set GLUON_BRANCH depending on build type

See merge request freifunk-region/stuttgart/site!3

Introduce the following build type:
| Build Type | When   |
|------------|--------|
| stable     | commit tag starts with "stable/" |
| beta       | commit tag starts with "beta/" |
| nightly    | trigger is a scheduled job |
| commit     | else |

Depending on the build type, set the GLUON_BRANCH accordingly:

| Build Type | GLUON_BRANCH  |
|------------|---------------|
| commit     | (unset)       |
| nightly    | nightly       |
| beta       | beta          |
| stable     | stable        |

CI: do not build all targets on all branches

See merge request freifunk-region/stuttgart/site!2

When building non-tagged and non-scheduled commits, only build
x86-64 to speed up the build.

create manifest after building

See merge request freifunk-region/stuttgart/site!1

This makes the gluon build fail since v2018.2.1

allow the SSID to change to the offline SSID once every 15 minutes

Citing the gluon v2018.1 release notes:
    This version changes the flash partition layout on some devices (TP-Link
    CPE/WBS 210/510). To avoid upgrade failures, make sure to upgrade to
    Gluon 2017.1.8 or the latest Gluon 2016.2.x (unreleased) before
    installing Gluon 2018.1.

So we need a yet unreleased ffs-1.4 version and have to bump the version
to 1.5 fot a release with gluon v2018.1.

In non multi domain setups, vxlan is mandatory.

Release notes cites:
--- snip ---
In single domain setups, the new mesh.vxlan option is mandatory. It
should be set to true in new meshes; existing setups should set it to
false to retain compatibility with older versions of Gluon.
--- snap ---

It seems that we do have to wait until all Nodes are upgraded to
v2018.1, so we set it to false by now.

Feature/connect to onboarder on shortcut

Merge branch 'feature/connect-to-onboarder-on-shortcut' of github.com:freifunk-stuttgart/site-ffs into feature/connect-to-onboarder-on-shortcut

Updated modules to a63d3c042cde5af6f702d98bb89b6a0ccc4d1cb9

With this update of the Gluon-Packages, we enable
freifunk-stuttgart/Gluon-Packages@a63d3c0

Switch to gluon v2017.1.8

Updated Jenkinsfile to use gluon v2017.1.8 instead of v2017.1.7.

Updated modules to a63d3c042cde5af6f702d98bb89b6a0ccc4d1cb9

With this update of the Gluon-Packages, we enable
freifunk-stuttgart/Gluon-Packages@a63d3c0

Provide Jenkinsfile to build firmware with jenkins

A few parameters lacked descriptions. These were added. Architectures do
not have a description yet.

As the current release does generate any artefacts to output/modules,
this folder will not be stashed or archived

As with the current release sunxi seems not to be supported, this
architecture will not be build by default