[raspberry] add base image

i currently try building it and then check if it works

README.md

@@ -53,6 +53,22 @@ to build a configuration run the following command (replace ```<hostname>``` wit
 nix build .#nixosConfigurations.<hostname>.config.system.build.toplevel
 ```
 
+### building raspberry image
+
+for the raspberry it is possible to build the whole configuration as an sd-card image which then can be flashed directly. more information about building arm on nixos can be found [here](https://nixos.wiki/wiki/NixOS_on_ARM).
+
+to be able to build the image you need to enable qemu emulation on the machine you are building with. therefore it is necessary to add the following to your configuration.nix:
+
+``` nix
+boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
+```
+
+then you can build the image with:
+
+``` shell
+nix build .#nixosConfigurations.rpi1_base_image.config.system.build.sdImage
+```
+
 ### run a configuration as vm
 
 to run a vm we have to build it first using the following command (replace ```<hostname>``` with the actual hostname):