drivers: create new directory

drivers: create new directory

Add a new top-level "drivers" directory to which all PMDs will be moved
for easier maintenance of both lib folder and drivers themselves. This
new directory is a dependency of all the apps in the app folder, so
the makefiles for each app are updated.
To the new top-level directory add a "net" subdirectory to classify
more specifically our existing PMDs as ethernet drivers

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
[Thomas: fix dependencies and merge several patches]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>