Travis now runs checkpatch.pl

Note that this implies:
- If checkpatch.pl failes, Travis results is now failed. In such a case,
  it must be checked that
  - checkpatch.pl fails for good reason (GP CamelCase style for ex.)
  - all the compilations steps are fine
- Signing the commit is now mandatory
     git commit -s ...
- When pushing several commits on the same branch (same pull-request),
  checkpatch.pl is run only on the code differences given by the
  the last commit

Signed-off-by: Pascal Brand <pascal.brand@st.com>