tar: do not build SELinux support for host variant

If we don't explicitly disable SELinux support in the host-tar build,
it might pick up system-wide installed SELinux libraries, causing the
tar in HOST_DIR/bin/ to depend on the host SELinux libraries, which is
not desirable to make the SDK portable/relocatable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>