add documentation about netbooting LibreELEC on Raspberry Pis

[wklaebe]

No description provided.

[bam80]

@HiassofT ?

[HiassofT]

First of all, thanks a lot for your contribution!

I'd prefer if you structure the wiki page a bit and focus on the LE specific bits:

Start with the general LE setup, i.e. boot/flash must be made available both via TFTP and NFS and disk/storage via NFS, and how to populate the boot/flash directory.

You can give short examples how eg /etc/exports or a TFTP config file look like, but oherwise drop the RPiOS specific bits like how to install specific TFTP server (there are several different ones available). Just add links to RPiOS forum threads or wiki pages where these bits are already discussed in detail.

As for kernel ip config: in general it's recommended to use kernel ip autoconfiguration, so ip=dhcp is enough - manual configuration is only needed in rare cases.

As RPi4 doesn't suffer from many of the limitations that RPi2/3 had it's best to add the few RPi2/3 specific bits in a separate section at the end. Eg mention here that TFTP server must be on the same server as DHCP - and better link to existing docs which describe how to setup a (proxy) DHCP