.gitlab-ci.yml: check flake8

Add a test to check Python code style in the whole buildroot tree.

Search files by type in order to help flake8 to find the Python scripts
without .py extension. But don't rely only in the output of 'file' as it
uses heuristics and sometimes it is wrong (specially identifying Python
files as C++ source for the 'file' version currently in the Docker
image).

Include in the output:
 - the list of Python files processed;
 - statistics for each kind of warning;
 - the total number of warnings;
 - the number of Python files processed.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>