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

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.