[CLEANUP] Remove unused and legacy files

* Delete all the content of the boards/ folder containing the job
configuration files for the different boards. The idea behind it is to
use a dynamic child pipeline from a dinamically created .yml file
(containing all the board jobs), thus simplifying maintenance,
avoiding copy and paste errors and allowing to dinamically define as a
global variable the list of machines to be built/deployed/testes.
* Delete files initially imported from SECO-NE original project and no
longer necessary.