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.