basic simulator: workaround: skip first simulation's frames

Due to some unknown bug in the eNB (or UE, or both), the first frames
are not correctly generated (or handled), which leads to a bad behavior
of the simulator in some cases (seen with 100 RBs: the UE reads a bad
MIB and switches to 25 RBs, which results in a deadlock in the
tcp_bridge_oai driver).

Let's "fix" this problem by skipping (in the driver) some frames at the
beginning of the simulation.