libpayload: usbmsc: Implement limited LUN support

libpayload: usbmsc: Implement limited LUN support

I always thought the support for multiple logical SCSI units in the USB
mass storage class was a dead feature. Turns out that it's actually used
by SD card readers that provide multiple slots (e.g. one regular sized
and one micro-SD). Implementing perfect support for that would require a
major redesign of the whole MSC stack, since the one device -> one disk
assumption is deeply embedded in our data structures.

Instead, this patch implements a poor man's LUN support that will just
cycle through all available LUNs (in multiple calls to usb_msc_poll())
until it finds a connected device. This should be reasonable enough to
allow these card readers to be usable while only requiring superficial
changes.

Also removes the unused 'protocol' attribute of usb_msc_inst_t.

BRANCH=rambi?,nyan
BUG=chrome-os-partner:28437
TEST=Alternatively plug an SD or micro-SD card (or both) into my card
reader, confirm that one of them is correctly detected at all times.

Original-Change-Id: I3df4ca88afe2dcf7928b823aa2a73c2b0f599cf2
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/198101
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
(cherry picked from commit 960534a20e4334772c29355bb0d310b3f41b31ee)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Change-Id: I39909fc96e32c9a5d76651d91c2b5c16c89ace9e
Reviewed-on: http://review.coreboot.org/7904
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>