Add a new "virtual disk" feature.

It's extremely difficult to test storage related logic on devices
that don't have physical SD card slots.  So to support better
debugging and testing, add a new "virtual disk" feature which mounts
a 512MB file through loop device.

It relies on the kernel having the "loop.max_part" value set to
something other than 0 via the boot command line, since that allows
all the existing partition logic to fall into place.

Bug: 34903607
Test: builds, boots, virtual disk works
Change-Id: I04c5b33e37319d867542985a56b7999a9b7cf35d
5 files changed