USB comms: part 4 towards @micolous PR #463

* make uart_communication(), storeCommand() and getCommand() static in comms.c
* move receiver thread creation and respective mutexes to comms.c
* add mutex and signal for tx buffer
* use comms.c for flasher as well
* remove comm functions from client/proxmark3.h
* this completes isolating all USB communication related functions in comms.c
* don't assume a port to be defined by a name. Change parameter in OpenProxmark() to void*
* comms.c: set sp and serial_port_name to NULL when offline