Allow ueventd to relabel block devices

There is a race in ueventd's coldboot procedure that permits creation
of device block nodes before platform devices are registered. In this case
the device node links used to compute the SELinux context are not known
and the node is created under the generic context: u:object_r:block_device:s0.

Ueventd has been patched to relabel the nodes on subsequent add events but
it needs permissions to be allowed to do it.

BUG=28388946

Change-Id: Ic836309527a2b81accc50df38bd753d54fa5e318
Signed-off-by: Mihai Serban <mihai.serban@intel.com>