Add USB serial support

This changeset makes the FlySight a composite USB device, supporting both the existing mass storage interface as well as a CDC (USB serial) interface. This directly exposes the u-blox module to the host, which by default yields a normal GPS stream of NMEA sentences, but being bi-directional allows the host to reconfigure the module in whatever manner is desired.

My Mac automatically gets a `/dev/tty.usbmodemfd121` supported by standard Apple drivers. Opening it gives:

    $GPVTG,,T,,M,0.811,N,1.503,K,A*2C
    $GPGGA,034532.00,4153.53396,N,08904.27424,W,1,11,1.08,242.1,M,-33.5,M,,*62
    $GPGSA,A,3,21,18,15,22,24,06,27,03,26,14,19,,1.84,1.08,1.50*03
    $GPGSV,3,1,12,03,16,301,10,06,24,284,22,14,15,223,19,15,49,052,24*73
    $GPGSV,3,2,12,18,72,327,27,19,09,324,30,21,74,185,26,22,40,288,32*7C
    $GPGSV,3,3,12,24,40,116,14,26,05,045,26,27,24,296,22,30,00,044,*72
    $GPGLL,4153.53396,N,08904.27424,W,034532.00,A,A*73