Enable L2TP and PPTP from upstream kernel

Android implementation of L2TP and PPTP (PPPoLAC and PPPoPNS) was
removed from Android kernel v4.14. AOSP projects (mtpd and ppp) were
modified correspondingly to use upstream kernel implementation of L2TP
and PPTP instead: [1,2]. Let's enable L2TP and PPTP from upstream kernel
so that modified mtpd/pppd projects can use upstream implementation
(without these, mtpd will fall back to Android implementations).

While at it, remove old Android drivers (PPPoLAC and PPPoPNS) from
kernel 4.4 and 4.9. Now that we enabled upstreamed implementation, we
are not going to use them anymore.

[1] https://android-review.googlesource.com/q/topic:pppolac+status:merged
[2] https://android-review.googlesource.com/q/topic:pppopns+status:merged



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Change-Id: I06d5c667999859b3e840219260582757101b11f9