Add missing unit test support files

Sample YAML files are needed inside the source
tarball for packaging unit tests.

Change-Id: I1b8a298be0fcd996540b7c2537c7524b412d7b3f
Preserve SELinux support in test images and tarballs

Allow master image devices to detect if the test image
can retain support for SELinux by checking the version of
tar installed in the master image.

Use SELinux options transparently elsewhere as the dispatcher
is known to support it on all supported LAVA installs.

Drop support if the test image has a version of tar --selinux
outputs "unrecognized" when called in the master image.

Change-Id: I978d7f6cf2c39e4e45f79c517fc48addfd0920da
cubieboard3: enable kernel dhcp for ramdisk booting

Change-Id: I85d048da1da505c620d3031b5d74c69884755746
cubieboard3: fix typo in boot_cmds_nfs

Change-Id: I70126fd5edfefea848b7d3b80bef6f117889e97d