usb communication (device side) refactoring

* merge cmd.c into usb_cdc.c
* move back usb_cdc.[ch] to common/
* declare low level functions usb_read() and usb_write() and more functions as static
* use cmd_receive() in bootrom.c and appmain.c
* remove unused memory wasting csrTab[100] in usb_cdc.c
* replace more byte_t by uint8_t
* more whitespace fixes