Linux802.15.4

How to configure Linux 802.15.4 interface for interconnectivity to Zephyr.

Reference hardware:

• "ATUSB" dongle on Linux side
• BOARD=frdm_kw41z on Zephyr side

Configuring Linux:

1. An older way to configure 802.15.4 interfaces/subsysem is lowpan-tools package (iz, etc. executable). That's what e.g. Ubuntu 16.04 ships. But it's not flexible enough for our use, e.g. it doesn't allow to configure 15.4 channel number and other similar params.
2. Instead, newer wpan-tools should be used, which allows to configure fine-grained params like above. On distros which lack native package, it can be built from source (tested on Ubuntu 16.04).
3. List available phy devices: iwpan list.
4. List available 15.4 interfaces: iwpan dev.
5. Configure channel to used by default by Zephyr: sudo iwpan phy wpan0 set channel 26.
6. Configure PAN ID to used by default by Zephyr: sudo iwpan dev wpan0 set pan_in 0xabcd.
7. wpan0 is an 15.4 interface, not suitable for IP networking directly (one sign of that is that it has 15.4 MTU, ~ 127). For IPv6 networking, we need to wrap it in 6LoWPAN interface: ip link add link wpan0 name lowpan0 type lowpan. Created lowpan0 interface will have the standard IPv6 MTU (1280), can have IPv6 addresses configured, etc.