config: make libarchive optional

config: make libarchive optional

The commit 66819e6 has introduced a dependency on libarchive to be able
to use some tar resources in the unit tests.
It is now an optional dependency because some systems do not have it
installed.

If CONFIG_RTE_APP_TEST_RESOURCE_TAR is disabled, the PCI test will not
be run. When a "configure" script will be integrated, the libarchive
availability could be checked to automatically enable the option.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Jan Viktorin <viktorin@rehivetech.com>