Add sources

source/firmware/shell.nix

@@ -0,0 +1,16 @@
+{ nixpkgs ? import <nixpkgs> {} }:
+
+let cross-rv5 = import <nixpkgs> {
+      crossSystem = {
+        config = "riscv32-none-elf";
+        gcc = { arch = "rv32i"; abi = "ilp32"; };
+        libc = "newlib";
+      };
+    };
+in
+cross-rv5.mkShell {
+  nativeBuildInputs = [ nixpkgs.gnumake nixpkgs.guile_3_0 ];
+  shellHook = ''
+    export NIX_SHELL_NAME="riscv"
+  '';
+}