Add stub code to read MS OS 2.0 descriptors.

Linux doesn't seem to actually support emitting them, so we can't check
that they're actually there, but commit the code to read them so that
if and when Linux gets support, it's easier to check.

This also adds some error checking for ill-formed binary object store
entries.

Test: check_ms_os_desc
Change-Id: I4baf6a16f0f96acbec468b843db65d4ab3a589fe