openpowerlink: use git repository instead of zip archive

Since commit 6c5c08b8,
openpowerlink package is rebuilt at every make call because
.stamp_downloaded is missing in the build directory.[D

The culprit is OPENPOWERLINK_EXTRACT_CMDS that remove and
replace the build directory.

unzip extract a directory "openPOWERLINK-V1.08.4" and
Buildroot expect a directory "openpowerlink-V1.08.4"

It is easier to use git repository instead of zip archive and
avoids rename the directory openPOWERLINK to openpowerlink.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>