Advanced bridge (snooping, stp)

[troglobit]

A basic bridge w/ multicast-snooping is defined in #83. It needs to be extended with a few advanced features to cover the minimal scope for our customers.

Tasks

• [x] Configurable IGMP/MLD querier support (#309)
• [x] bridge stp|rstp, with mstpd
• [ ] #520
• [ ] Control BUM flooding to CPU
• [ ] MRP multicast quick-reconnect