USB comm: prepare for @micolous change (PR#463) (#587)

* move communication related code from proxmark3.c and cmdmain.c to new file comms.c
* replace byte_t by uint8_t in uart_posix.c and uart_win32.c
* move OpenProxmark() and CloseProxmark() from flasher.c to flash.c
* move print_lock mutex including initializer to ui.c
* minor changes in printing help texts
* no changes in comms functionality yet