USB gadget 101 tutorial, add more references and details

[orangecms]

We have a brief section in the README, but it needs more details.

add reference: https://www.kernel.org/doc/html/latest/driver-api/usb/gadget.html

networking

[x] Include configuring an IP address, be it via DHCP or statically.
The latter is currently not yet offered by cpud itself. See #312.
Note that cpud would also spawn a shell, and that could in turn run an rc script, but that becomes complex for the end user and build process.

So far, cpud calls into external programs.
[ ] Note that the rootfs will need a DHCP client (dhclient) or an ip (or similar) command.

[ ] Also mention the possibility to use NFS now and the direct 9p over USB driver (usb9pfs) that Pengutronix added, though it would need corresponding userspace setup code.
https://pengutronix.de/de/blog/2025-01-22-pengutronix-at-fosdem-2025.html

[x] USB gadgets should have static MAC addresses for consistency. Then a laptop on the other side can rely on its own static counterpart config.

[x] Note that USB ethernet vs CDC ethernet+serial require different cmdline args:
g_ether.dev_addr+g_ether.host_addr vs g_cdc.dev_addr + g_cdc.host_addr

drivers

[x] Remove RNDIS from the suggested drivers.

Not fully confirmed, may be a bug in the older vendor kernel I had in this instance: I found it to clash with the regular ethernet emulation driver, resulting in network not working. A usb0 device would show up, and with IP addresses assigned on both sides, no ping would respond.

RNDIS has a long story of issues and attempts to be dropped entirely.
https://www.phoronix.com/news/Linux-RNDIS-Removal-EOY2024