Mandate 'Signed-off-by' line in commit messages ...

This patch updates the configuration file for the checkpatch.pl
script to check for the presence of a 'Signed-off-by' line in the
commit message. This is now required by TF contribution process.

Change-Id: I8bcd6d39a5dcd85547244d60c10a7cd2148c404f

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

Move checkpatch options in a configuration file ...

At the moment, the top Makefile specifies the options to pass to the
checkpatch script in order to check the coding style. The checkpatch
script also supports reading its options from a configuration file
rather than from the command line.

This patch makes use of this feature and moves the checkpatch options
out of the Makefile. This simplifies the Makefile and makes things
clearer.

This patch also adds some more checkpatch options:
  --showfile
  --ignore FILE_PATH_CHANGES
  --ignore AVOID_EXTERNS
  --ignore NEW_TYPEDEFS
  --ignore VOLATILE
The rationale behind each of these options has been documented
in the configuration file.

Change-Id: I423e1abe5670c0f57046cbf705f89a8463898676