init: enable init to relabel symlinks for system_block_devices

early mounted block device are created by 'init' in its first stage, so
the following restorecon() now finds device nodes and their corresponding
symlinks. The CL adds rule to make sure the block and
system_block_devices can be relabeled by init in this case.

Bug: 35792677
Bug: 27805372

Test: tested ota using 'adb sideload' on sailfish

Change-Id: I7d9d89878919c1267bf3c74f0cdbb4367b5ad458
Signed-off-by: Sandeep Patil <sspatil@google.com>