devtools: pass custom options to checkpatch

devtools: pass custom options to checkpatch

Add the ability to pass custom options to checkpatch script. An example
of use is to change the output format so it can run in emacs compilation
mode:

  DPDK_CHECKPATCH_PATH=/path/to/linux/scripts/checkpatch.pl \
    DPDK_CHECKPATCH_OPTIONS='--emacs --showfile --no-color' \
    /path/to/dpdk.org/devtools/checkpatches.sh

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>