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

In order to build our firmware with jenkins, we need a jenkins pipeline
script. With this commit, this script is provided. Documentation can be
found within the Jenkinsfile.

missing commas, other minor changes

"Welcome *to*" and only single blank

Option to override build date

Setting BUILD_DATE overrides the date used in the generated image names.
With this it is possible to have a consistent build date across all
architectures, making it easier to identify a release. Furthre more it
might be the key to have reproducible builds