Introduce new filter mechanism to improve robustness against storms on network

The already existing netload filters against storms shall be replaced by a single filter module. This module determines if a packet shall be dropped or processed.

A callback functions with the following parameters shall be implemented:

• user callback pointer
• current load of unprocessed frames
• current system timer ticks

The callback must return true if the frame shall be processed and false if the frame shall be dropped.

The TCP Stack will provide a default filter as global symbol, which have a similar behavior as the existing filters.