genfs_contexts

Right now usbfs doesn't have any labels, generating the
following kernel warnings:

<7>[    3.009582] SELinux: initialized (dev usbfs, type usbfs), not configured for labeling

and the occasional SELinux unlabeled auditallow logs:

<4>[  285.579254] type=1400 audit(1402010345.094:16): avc: granted { search } for pid=371 comm="qcks" name="/" dev="usbfs" ino=15794 scontext=u:r:kickstart:s0 tcontext=u:object_r:unlabeled:s0 tclass=dir
<4>[  285.632354] type=1400 audit(1402010345.154:18): avc: granted { search } for pid=371 comm="qcks" name="001" dev="usbfs" ino=15796 scontext=u:r:kickstart:s0 tcontext=u:object_r:unlabeled:s0 tclass=dir

Make sure usbfs is assigned via genfscon

Change-Id: I7191f2584014ba55a3c3a98e7efd0350dc958782