Adapt xdpd for DPDK 1.7

In DPDK 1.7, Poll Mode Drivers (PMD) register from static
constructors and there is no rte_pmd_init_all() function anymore.
rte_eal_pci_probe() has also become optional.
In order for PMDs to register, their archive must be force-included
in the xdpd binary, which is achieved with --whole-archive linker
flag.

Signed-off-by: David Verbeiren <david.verbeiren@intel.com>
Merge remote-tracking branch 'origin/devel0.6/dpdk1.7' into devel-0.6
Updating documentation