eal: support driver loading from directory

eal: support driver loading from directory

Add support for directories as arguments to -d for loading all drivers
from a given directory. Additionally a default driver directory can be
set in build-time configuration, in which case it will be always be used
when EAL is initialized.

This simplifies usage in shared library configuration significantly over
manually loading individual drivers with -d, and allows distros to
establish a drop-in driver directory for seamless integration
with 3rd party drivers etc.

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