rlcmac: Implement UL TBF Countdown procedure

The Countdown procedure (decreasing CV field in UL RLCMAC data blocks)
is defined in TS 44.060 9.3.1.

It is implemented by means of counting/calculating the amount of RLC/MAC
blocks to be transmitted based on the LLC frames in the several LLC
queues, without actually generating the RLC/MAC blocks. The blocks
cannot be generated ahead of time because that wouldn't allow recreating
them in case Tx UL CS changes, or if a higher priority LLC frames
arrives.
The functions calculating the required amount of RLC/MAC blocks are
coded so that they early return to avoid spending more time than
necessary counting blocks (< BS_CV_MAX).
An extra heuristic optimization to be used when LLC queues are long is left
documented as a TODO, in order to test further the general non-optimized
path for now. Once the counting is proved to work reliably, that and
other heuristic optimizations ca be implemented, like keeping and
decreasing CV cached counter while no Tx CS change occurs or no new LLC
frames are enqueued.

Change-Id: If043c86a0c2b89d0ac0b8174de39fbcb22bed8cb
---
M include/osmocom/gprs/rlcmac/llc_queue.h
M include/osmocom/gprs/rlcmac/rlcmac_enc.h
M include/osmocom/gprs/rlcmac/tbf_ul.h
M src/rlcmac/llc_queue.c
M src/rlcmac/rlcmac_enc.c
M src/rlcmac/tbf_ul.c
M tests/rlcmac/rlcmac_prim_test.c
M tests/rlcmac/rlcmac_prim_test.err
M tests/rlcmac/rlcmac_prim_test.ok
9 files changed, 518 insertions(+), 66 deletions(-)