sepolicy: init: Allow init to access custom block device labels

During restorecon init needs readlabel access to block devices. It
already has access to the generic "block_device", but prints out
errors because it is not able to access devices with custom labels.
Updated the policy to allow the same.

Change-Id: Icb19e9d7563201b8d7585c05e3a00be1d73a7c30