devtools: add script to get maintainers from patch

This is a wrapper to Linux kernel get_maintainer.pl file and only
supports parsing MAINTAINERS file (no git fallback etc..)

Requires DPDK_GETMAINTAINER_PATH devel config option set, please check
devtools/load-devel-config.

DPDK_GETMAINTAINER_PATH should be full path to the get_maintainer.pl
script, like:
DPDK_GETMAINTAINER_PATH=~/linux/scripts/get_maintainer.pl

Can be used individually:
./devtools/get-maintainer.sh <my.patch>

Or via git send-email, to add maintainers automatically:
git send-email --to-cmd ./devtools/get-maintainer.sh \
--cc dev@dpdk.org HEAD -4

Currently there is an ugly workaround to be able to use Linux script out
of the kernel tree, later better method can replace it.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>