l2tp: convert to gentargets and change to xl2tp

l2tp is no longer being developed, and xl2tpd is forked from l2tpd and is
maintained by Xelerance Corporation.

[Peter: remove invalid pppd dependency]
Signed-off-by: Martin Banky <Martin.Banky@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>