msg->l2h should always point at the first byte of the header after the
generic simtrace header. For some reason this doesn't work if used
globally, but it is required for the MSGC_MODEM messages to be
interpreted correctly.
The current protocol was card-emulation specific. The new protocol is
generic/flexible enough to accommodate both tracing and card emulation,
as well as modem control and other future extensions.
We now generalize the USB communiction and abandon the 'req_ctx'
structure inherited from openpcd. Instead we use the libosmocore 'msgb'
structure to handle incoming and outgoing USB tranfers. We also use
linuxlist-based msgb-queues for each endpoint.
