eal: add legacy memory option

eal: add legacy memory option

This adds a "--legacy-mem" command-line switch. It will be used to
go back to the old memory behavior, one where we can't dynamically
allocate/free memory (the downside), but one where the user can
get physically contiguous memory, like before (the upside).

For now, nothing but the legacy behavior exists, non-legacy
memory init sequence will be added later. For FreeBSD, non-legacy
memory init will never be enabled, while for Linux, it is
disabled in this patch to avoid breaking bisect, but will be
enabled once non-legacy mode will be fully operational.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>