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.