net: introduce a network packet filter framework

This provides the infrastructure to create network packet filter rules
and to apply them to the RX and TX packet paths. Rules are made of
simple condition tests that can be linked together, creating a facility
similarly to the Linux iptables functionality.

A couple of generic and Ethernet-specific condition tests are also
provided.

Additional tests can be easily created on top of this.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>