eal: make the -n argument optional

eal: make the -n argument optional

Obtaining the correct value of memory channels, especially from a
running system, can be anything from difficult to plain impossible.
Since the value is merely an optimization and does not affect functionality
otherwise, its pointless to force such a guess on users initially, such
things belong to performance tuning phase.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: David Marchand <david.marchand@6wind.com>