Custom XNCP protocol commands (#57)

* [TEST] Multicast packet filter

* Use the correct filter

* Implement manual source routing

* [no ci] Age out routes after a single use

* Clean up naming and fix bitshifts

* Move XNCP configuration into its own file

* Don't grow Ember buffer, the application crashes at runtime

* Prefix XNCP config

* Allow overriding the board and manufacturer name in-firmware

* Compute the length separately to isolate payload additions

* Don't require an enable flag

* Remove `_OVERRIDE` suffix from board and manuf name

* Use a status and a command ID for all responses

* Avoid scope issues with curly braces

* Use a generic command for manufacturing token overrides

* Fix command parsing for `SET_SOURCE_ROUTE_REQ`

* WIP: Disable automatic source routing and inject source routes directly

* WIP: Disable the packet handoff component and go lower

* Revert back to source routing override until GSDK bugfix

* Remove routing table modifications

* Reorder `#define`s for readability

* [TEST] Log all known counters

* Revert "[TEST] Log all known counters"

This reverts commit c13f5f9.

* Fix compile-time warning about missing XNCP functions

* Actually use `get_manual_source_route`

* Migrate `app.c` to `app.cpp`

* Revert "Migrate `app.c` to `app.cpp`"

This reverts commit 591bd07.

* Implement a firmware build string command

* Correctly set the relay index and handle empty source routes

* Insert a fake entry into the routing table to source route a single hop

* Pick routing table entry at random when forcing a direct route

* Expose the current date in templates

* Add a new XNCP command to return the firmware flow control settings

* Add `FEATURE_FLOW_CONTROL_TYPE` to the feature bitmap