fs/common.mk: set SELinux file security contexts

Set the SELinux file security contexts using setfiles when generating
root filesystem images.

Without such security contexts created at build time, they need to be
setup at first boot by running the restorecon utility on the target.
This has two drawbacks:

 - You have to special case the first boot, which cannot be done in
   enforcing mode, and will have to run restorecon, then reboot.

 - You cannot support read-only filesystems.

By setting up the security contexts at build time, we can have a
filesystem image that is immediately ready to boot an SELinux system
in enforcing mode, including if the root filesystem is read-only.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>