samples: net: Use qemu_x86 config file by default for echo apps

As the default board is qemu_x86, make sure there is similarly
named config file that can be used by default.

Change-Id: Ibc480d3b4eecc2601e3670c8da19be9c01833c36
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>