Add templates to allow easy inheritance of Ghaf

[brianmcgillion]

Ghaf is a framework to allow the easy creation of
virtualized edge devices. As such it should be
used to derive projects that are configured to
the specific use cases.

These templates allow for that easy creation.

[vilvo]

Could we develop this approach so that we'd avoid <PROJ NAME> renaming separately here https://github.com/tiiuae/ghaf/pull/164/files#diff-cb02caf321218f41518829b1217328404d838b7caf4832abdf5cb46e9b148881R25

nix flake supports templates with both new and init allow project folder (name) as parameter and further

--arg name expr

Pass the value expr as the argument name to Nix functions.

--argstr name string

Pass the string string as the argument name to Nix functions

which could probably be used for this renaming instead.

[vilvo]

--arg name expr

Pass the value expr as the argument name to Nix functions.

--argstr name string

Pass the string string as the argument name to Nix functions

which could probably be used for this renaming instead.

I checked that my proposal seems not supported NixOS/nix#4017 (comment)

I also tested the PR templates with:
nix flake new --template github:brianmcgillion/ghaf/templates#target-x86_64-generic ~/generic_ghaf
and it works like it says in the documentation.