rlcmac: Introduce lower primitive layer API

This primitive API allows the application to drive the RLCMAC layer
based on a lower L1CTL interface.

* RACH req are requested to the lower layers by the RLCMAC through the
  RACH.req primitive.
* Received ImmAss, SI are fed to the RLC/MAC layer through CCCH_DATA.ind
* The RLCMAC registers DL and UL TBFs in the lower part (firwmare,
  L1CTL):
** The lower part will trigger PDCH_RTS.ind in every TS in the
  "(dl_slotmask|ul_slotmask)" superset, with the received USF. This
  allows UL uplink to transmit blocks when requested by USF, or UL/DL TBF
  to answer polls from PCU at a specific N+X FN (RRBP). This also allows
  the RLCMAC layer to update its GSM clock and trigger internall
  timeouts.
** The lower part will trigger PDCH_DATA.ind in every TS in the
  dl_slotmask if either a CTRL block is received or a data block
  matching the configured dl_tfi is received.
** Upon receival of PDCH_DATA.ind, the RLCMAC layer updates its FN clock
  and submits PDCH_DATA.req based on USF.

This commit only adds the primitive set and API to submit/receive them,
but doesn't really implement them yet internally.

Related: OS#5500
Change-Id: I3d97425ec75059ceae983de869419230e8c4df01
---
M include/osmocom/gprs/rlcmac/rlcmac_prim.h
M include/osmocom/gprs/rlcmac/rlcmac_private.h
M src/rlcmac/rlcmac_prim.c
M tests/rlcmac/Makefile.am
A tests/rlcmac/rlcmac_prim_test.c
A tests/rlcmac/rlcmac_prim_test.err
A tests/rlcmac/rlcmac_prim_test.ok
M tests/testsuite.at
8 files changed, 478 insertions(+), 4 deletions(-)