This does some basic formal testing, just like done for packages.git.
Developers can test their commits locally by running `make
ci-formal-checks`. If working on another branch than `master`, add
`BRANCH=<branch>`. Later compiling of device tree files, checking kmods
or other more specific tests could be done. However this is a first step
to CI.

Signed-off-by: Paul Spooren <mail@aparcar.org>