build: move variable for drivers to top level

build: move variable for drivers to top level

If for debugging we disable the driver directory in the meson.build file,
we get an error because the variable "driver_classes" does not exist.
This is because driver_classes is only defined in the
drivers/meson.build file. Defining driver_classes in dpdk/meson.build file
will make it easier for compiling separate directories.

In the process, we rename driver_classes to dpdk_driver_classes for
consistency with the other variables.

Signed-off-by: Andrius Sirvys <andrius.sirvys@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>