rust: Make cargo warn when missing env variable

Adding a build script impacts the metadata of the crate which means that
it also impacts all the mangled function names and eventually has a
minor impact on the final binary. All rust symbols end with a hash of
the crate metadata.

src/rust/bitbox02-rust/build.rs

@@ -0,0 +1,26 @@
+// Copyright 2024 Shift Crypto AG
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Emit a warning if FIRMWARE_VERSION_SHORT isn't set. We don't want this to be a hard error during
+// development so that rust tools are happy.
+fn main() {
+    let version = option_env!("FIRMWARE_VERSION_SHORT");
+    if let Some(version) = version {
+        if version.is_empty() {
+            println!("cargo::warning=FIRMWARE_VERSION_SHORT is empty");
+        }
+    } else {
+        println!("cargo::warning=FIRMWARE_VERSION_SHORT is not set");
+    }
+}