openl2tp: add new recipe

OpenL2TP is an open source L2TP client / server, written specifically
for Linux. It has been designed for use as an enterprise L2TP VPN server
or in commercial, Linux-based, embedded networking products and is able
to support hundreds of sessions, each with different configuration.
It is used by several ISPs to provide L2TP services and by corporations
to implement L2TP VPNs.

Signed-off-by: Li Xin <lixin.fnst@cn.fujitsu.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>