Fix "can't find crate for std" for stm32-metapac-gen deps.

stm32-metapac-gen/src/assets/build.rs

@@ -0,0 +1,16 @@
+use std::env;
+
+fn main() {
+    let _chip_name = env::vars_os()
+        .map(|(a, _)| a.to_string_lossy().to_string())
+        .find(|x| x.starts_with("CARGO_FEATURE_STM32"))
+        .expect("No stm32xx Cargo feature enabled")
+        .strip_prefix("CARGO_FEATURE_")
+        .unwrap()
+        .to_ascii_lowercase();
+
+    #[cfg(feature = "rt")]
+    println!("cargo:rustc-link-search=src/chips/{}", _chip_name);
+
+    println!("cargo:rerun-if-changed=build.rs");
+}

stm32-metapac-gen/src/assets/lib_inner.rs

@@ -0,0 +1,6 @@
+// GEN PATHS HERE
+mod inner;
+
+pub mod common;
+
+pub use inner::*;