build(f722): centralize linker layout

firmware/boards/geprc-f722-v2/build.rs

@@ -1,9 +1,11 @@
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rerun-if-changed=memory.x");
-    println!("cargo:rustc-link-arg=-Tlink.x");
+    let layout = std::path::Path::new(&std::env::var("CARGO_MANIFEST_DIR").unwrap())
+        .join("../../platforms/stm32f7/link/f722");
     println!(
-        "cargo:rustc-link-search={}",
-        std::env::var("CARGO_MANIFEST_DIR").expect("Cargo must set CARGO_MANIFEST_DIR")
+        "cargo:rerun-if-changed={}",
+        layout.join("memory.x").display()
     );
+    println!("cargo:rustc-link-arg=-Tlink.x");
+    println!("cargo:rustc-link-search={}", layout.display());
 }