[nokia-n900] Add swiss german keyboard layout

This PR adds the swiss german keyboard layout to the nokia-n900 package.

The layout is selectable directly in pmbootstrap init when you select the nokia-n900 device.

This layout is only for the keyboard in the console. When using X11 there is by default the us layout because of this config file: https://github.com/postmarketOS/pmbootstrap/blob/3510a4868fd67677ac5c781de3d814a6072253c6/aports/device/device-nokia-n900/keymaps/40-xkb.conf#L5-L6

It would be nice if the selected layout will also be set for X11 by default, but I don't know how to handle it.

I've made the mapping myself by comparing the default layout configured in Maemo for the swiss german and I added some additional combination for the missing keys. (list of available keycodes here: https://deskthority.net/wiki/Loadkeys#Symbols_List)

I've also created a reference image that I'll also add it to the N900 wiki page also.