devShells.zephyr: allow disabling west, adding zephyr source

shells/zephyr/default.nix

@@ -1,24 +1,30 @@
-{ mkShell, zephyr, cmake, ninja, gnumake, python3
-, toolchains ? with zephyr.toolchains; [ arm-zephyr-eabi ] }:
+{ lib, mkShell, zephyr, cmake, ninja, gnumake, python3
+, toolchains ? with zephyr.toolchains; [ arm-zephyr-eabi ], enableWest ? false
+, zephyrSrc ? null }:
 let zephyr-sdk = zephyr.sdk.override { inherit toolchains; };
 in mkShell {
-  packages = [ cmake ninja gnumake python3 ] ++ (with python3.pkgs; [
-    # requirements-base.txt
-    pyelftools
-    pyyaml
-    pykwalify
-    canopen
-    packaging
-    progress
-    psutil
-    pylink-square
-    pyserial
-    requests
-    anytree
-    intelhex
-    west
-  ]);
+  packages = [ cmake ninja gnumake python3 ] ++ (with python3.pkgs;
+    [
+      # requirements-base.txt
+      pyelftools
+      pyyaml
+      pykwalify
+      canopen
+      packaging
+      progress
+      psutil
+      pylink-square
+      pyserial
+      requests
+      anytree
+      intelhex
+    ] ++ lib.optional enableWest python3.pkgs.west);
 
   ZEPHYR_TOOLCHAIN_VARIANT = "zephyr";
   ZEPHYR_SDK_INSTALL_DIR = "${zephyr-sdk}";
+} // lib.optionalAttrs (zephyrSrc != null) {
+  ZEPHYR_BASE = "${zephyrSrc}";
+  shellHook = ''
+    PATH="${zephyrSrc}/scripts:$PATH"
+  '';
 }