eal: warn on legacy memory allocation requirement

eal: warn on legacy memory allocation requirement

When using --no-huge mode, dynamic allocation is not supported.
Because of this limitation, the option --legacy-mem is implied
and -m may be needed to specify the amount of memory to allocate.
Otherwise the default amount MEMSIZE_IF_NO_HUGE_PAGE will be allocated.

The option --socket-mem can also be used with --legacy-mem
when hugepages are supported.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>