SW/Docs: Sub-G 802.15.4 Greybus demo

[jadonk]

• [x] Connect at Sub-G radio frequency
• [x] Minimal patching for Linux (CC1352 to register with Linux WPAN subsystem)
• [x] Linux host demo application that forms an 802.15.4 PAN (Personal Area Network) with coordinator node allows RFD to join PAN
• [x] enumerates remote SPI & I2C via 6LowPan & Greybus
• [x] performs remote SPI & I2C transactions on simple, readily available SPI & I2C peripherals via 6LowPan & Greybus

[cfriedt]

LIkely will remove the requirement for coordinator. It's unnecessary for 6LowPAN and currently not anywhere close to being implemented on the Linux side. IEEE 802.15.4 joining isn't even necessary to use 6LowPAN over 15.4.

The host demo application is essentially the same as that for BLE and 2.4GHz 15.4.

This ticket should probably also be a Milestone.

There are really 2 remaining issues to complete this Milestone:

• zephyrproject-rtos/zephyr#26312 (PR: zephyrproject-rtos/zephyr#26316)
• zephyrproject-rtos/zephyr#26315 (PR: zephyrproject-rtos/zephyr#29598)

[cfriedt]

Video of SubGHz Blinky

https://twitter.com/cfriedt/status/1324757273026154496

I2C transactions over SubGHz:

Just working on SPI now.

[jadonk]

@lorforlinux @vaishnavachath I think we can almost close this, but we need to

1. re-open the upstream pull-request for BeagleConnect Freedom (BCF),
2. align with upstream on adding the Greybux-for-Zephyr module and
3. get this demo documented in-tree (.rst files per Zephyr documentation).

One of the discussions with the GSoC students got into creating a Posix target for Greybus-for-Zephyr.

I think we need a combination of:

1. a posix board target for Greybus-for-Zephyr and
2. a stand-alone test bench to perform various bus actions over virtual serial and network sockets

With these 5 items, we should be able to finally really close this issue.

Check items:

• [ ] BCF upstream provides stable SubG and 802.15.4
• [ ] Greybus-for-Zephyr module linked upstream provides Greybus
• [ ] Test bench provides stable toggle demo
• [ ] Documentation with in-tree .rst files provides stable demo documentation