The image builder will currently emit a warning if a NoC-block has
unconnected ports. This requires specifying `-W` to continue building,
which is the intended behaviour.
However, there are cases where there are legitimate reasons to not
connect a block, e.g., in the X310, where the radio block has 2x2 ports,
but we only connect 2 RX ports, and 1 TX port.
This adds a way to declare a "null" connection, which makes it explicit
in the image core YAML that a connection was deliberately omitted. Such
connections are created by connecting to a port called "_none_", e.g.:
```
connections:
- { srcblk: _device_, srcport: _none_, dstblk: radio0, dstport: in_1 }
```
The block name is irrelevant, but the schema requires providing a block
name.
This commit also amends the x3xx_radio_base.yml to avoid warnings when
building X3x0 bitfiles.
Original-commit: 6758966a5ac127a396e8b3b39f96aa484098e50f
Martin Braun
fba65c68f7