With this the user can select multiple logical units to expose over USB MSC
at once, eg: pyb.usb_mode('VCP+MSC', msc=(pyb.Flash(), pyb.SDCard())). The
default behaviour is the original behaviour of just one unit at a time.
Eventually these responses could be filled in by a function to make their
contents dynamic, depending on the attached logical units. But for now
they are fixed, and this patch fixes the MODE SENSE(6) responses so it is
the correct length with the correct header.
SCSI can support multiple logical units over the one interface (in this
case over USBD MSC) and here the MSC code is reworked to support this
feature. At this point only one LU is used and the behaviour is mostly
unchanged from before, except the INQUIRY result is different (it will
report "Flash" for both flash and SD card).
