Travis now runs checkpatch.pl

Note that this implies:
- If checkpatch.pl fails, 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

Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org>
Signed-off-by: Pascal Brand <pascal.brand@st.com>