Checkpatch: Use local copy of checkpatch

Avoid checking out the checkpatch.pl files from GitHub as
that requires network access and might be subject to
instability. Align to what other projects do in the Open CI
and host checkpatch.pl directly.

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I1a48d07a03673b2fafe89eeee06938c1cf9c8961
4 files changed