Custom XNCP protocol commands

[puddly]

This PR introduces custom XNCP commands to extend EmberZNet with useful functionality:

• EmberZNet filters traffic for multicast groups the coordinator is not explicitly a member of. This PR implements a packet filter to bypass this limitation.
• Manually specifying source routes is not possible with EmberZNet. The firmware is customized to allow this.
• Override the board and manufacturer names (with a custom command).
• Support arbitrary version suffix strings.
• Return the flow control settings for the firmware.

To facilitate future firmware extensions and to allow applications communicating with the firmware to query these firmware extensions, I've implemented a simple protocol using EZSP's customFrame command.

[MattWestb]

One PM: manually setting one source router is making problems if the rout is being broken and the network cant healing therefor its normally automatic or no source routing being used.

[puddly]

Right now, sendUnicast in EmberZNet doesn't let you specify a source route when sending a packet, unlike Z-Stack and deCONZ. For Z-Stack with zigpy-znp, I use source routing as one of the retrying methods in case normal routing fails. I would like to use it the same with with bellows so that this logic can be moved into zigpy.

For zigpy's source routing, we assume symmetry and just use the last-taken route from a device as the source route when sending to the device. This can definitely be improved (speculative routing, randomization, better discovery, etc.).

[puddly]

New feature for firmware builders: overriding the board name. This uses a custom XNCP command so it will require support in bellows but it will allow you to override the firmware name that is displayed within Home Assistant:

c_defines:
  XNCP_BOARD_NAME: '"My Board v1.0"'
  XNCP_MANUF_NAME: '"Some Manufacturer"'

Currently, the board and device name are stored in USERDATA as manufacturing tokens and cannot be easily rewritten.

[puddly]

The bellows side of this is done, just needs unit tests (zigpy/bellows#611).

[evelant]

@puddly Is there an easy way to build this for the sonoff-e dongle as well? I'm wondering if it might help with the group command issues in home-assistant/core#86411 edit: sorry I meant zbdongle-e, I always get them mixed up

[puddly]

I'm going to go ahead and merge this. Without software explicitly running the custom commands, the only user-visible change will be arbitrary group packets "just working".