boot/stm32: autodetect thumbv6, remove cargo feature.

embassy-boot/stm32/build.rs

@@ -24,4 +24,9 @@ fn main() {
     if env::var("CARGO_FEATURE_DEFMT").is_ok() {
         println!("cargo:rustc-link-arg-bins=-Tdefmt.x");
     }
+
+    let target = env::var("TARGET").unwrap();
+    if target.starts_with("thumbv6m-") {
+        println!("cargo:rustc-cfg=armv6m");
+    }
 }