Relabel filesystem if SELinux is available

Relabel the filesystem during image builds if SELinux is supported
in the kernel of the build machine and userspace tools are available.

Otherwise touch /.autorelabel to schedule a relabel the first time
the image boots. We relabel when possible because it decreases first
boot time.

Change-Id: I0bec885d6e5d4f4e1106f3bd2a90ba5f86395b07
Partial-Bug: 1347845
Merge "Relabel filesystem if SELinux is available"