Create a label for the root block device.

Change-Id: If4de8d3515727c0b2f95c88c1125410d9894a9ba
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>

device.te

@@ -61,3 +61,6 @@ type hci_attach_dev, dev_type;
 # All devices have a rpmsg device for
 # achieving remoteproc and rpmsg modules
 type rpmsg_device, dev_type;
+
+# Partition layout block device
+type root_block_device, dev_type;