For the initialization of the xran library (split 7.2), we need some
parameters such as PRACH or frame config. However, to date,
initialization of split at level 7 use the split 8 openair0_config
structure, which does not have the necessary information.

Hence, add a single structure that contains that information.

* make a single call to various function
* make fill_rf_config() static, it is only used in this file
* do not initialize the number of antennas in openair0_cfg in
  init_NR_RU(), as it is already done in fill_rf_config()

Harmonize the call to openair0_transport_load(), by putting it in a
single place instead of multiple calls

Further, delay the call to ru_thread, for two reasons:

1) it is somewhat "symmetric" with local RF, which is done there as well

2) for the xran library, we need to read some parameters such as PRACH,
   frame format, etc. At the RU, the corresponding FAPI config request
   parameter structure is copied from the gNB to RU at the beginning of
   the ru_thread, so we have to wait for that to happen.

init_RU_proc() does not start the ru_thread, this is done by
start_RU_proc()

(don't use sample app)

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