Changes

RTP: add vty configuration for twjit

This patch is the first step in the series adding the ability to use
twrtp+twjit instead of Belledonne ortp.  twjit requires vty config
for tunable parameters - add this config.  Actual use of this added
config will happen in subsequent patches.

Related: OS#6474
Change-Id: I9ccd060fce8416e01356fccd9ae465a7298a9ce5

RTP: implement RTP socket abstraction layer

This abstraction layer is another step toward supporting a choice
between legacy Belledonne ortp and Osmocom-integrated twrtp.

Related: OS#6474
Depends: Ibda74c0dbfb163f5d0e3fb13f593a6e2c6817673 (libosmo-netif.git)
Change-Id: Ic945f6d753ec7f5c0ac5cecc9f71049464de7e8f

RTP: use new abstraction layer instead of direct osmo_ortp

As of this patch in the series, the RTP library in active use
is still always ortp, but all BTS code now accesses it through
the new abstraction layer, paving the way for twrtp option.

Related: OS#6474
Change-Id: I5e3de4f84e8e8875437cb94a9b8feeb07f9cc1cf

RTP: add vty option for ortp vs twrtp selection

With this patch it finally becomes possible for the user to select
which RTP library should be used: ortp or twrtp.  ortp is still
the default for now, in order to not alter behavior for existing
installations until twrtp receives more real world testing by users
beyond the original author.  Future patches may change the default
or even remove ortp support altogether - but twrtp should receive
more testing as a user opt-in feature first.

Related: OS#6474
Change-Id: Iff4e3a399250c16ba8fe4cb12e4e22f4c6b346ec

RTP: make ortp optional at compile time

It is now possible to run osmo-bts configure with --disable-ortp
option, which makes it use only twrtp library (integrated into
libosmo-netif) and removes all dependency on Belledonne software,
such that osmo-bts can be built on top of libosmo-abis that has also
been configured with its respective --disable-ortp option.

TODO: contrib/ber/rtp_ber utility still uses ortp and has no support
for twrtp or any other alternative - therefore, when osmo-bts is
configured with --disable-ortp, this utility is not built.

Related: OS#6474
Change-Id: Ib171bd42a65117457319befee2615e0c36c8d9e1

remove bogons from src/osmo-bts-oc2g/Makefile.am

This Makefile.am contained references to ORTP_CFLAGS and ORTP_LIBS,
which are bogons because our configure makes no pkg-config calls
for ortp, whether ortp support is included or excluded - instead
these pkg-config calls that produce ORTP_CFLAGS and ORTP_LIBS
happen only in libosmo-abis, while all subsequent ortp users in
Osmocom go through osmo_ortp shim provided by that library.

Related: OS#6474
Change-Id: Id9f091a42118393f973b982e18d1920f72bfcbb4