Instead of having a single config specifying the memory pool size for
variable-sized net buffers, have a separate one for TX and RX for better
configuration granularity when optimizing memory usage of the
application.
Deprecate the old configuration but use its value as a default (for now)
for the new configs. This will need to change when the config is
deleted.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
8e2722e1ca