refactor: move firmware workspace under firmware

firmware/boards/kakute-f7-mini-v3/build.rs

@@ -0,0 +1,9 @@
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=memory.x");
+    println!("cargo:rustc-link-arg=-Tlink.x");
+    println!(
+        "cargo:rustc-link-search={}",
+        std::env::var("CARGO_MANIFEST_DIR").expect("Cargo must set CARGO_MANIFEST_DIR")
+    );
+}