integration-test: fixes in build.rs

Not exactly sure why this distinction is necessary. But by looking at the
verbose cargo output, I found out that build.rs never found link.ld

integration-test/bins/multiboot2_chainloader/build.rs

@@ -1,5 +1,5 @@
 fn main() {
-    let linker_script = "multiboot2_chainloader/link.ld";
+    let linker_script = "link.ld";
     println!("cargo:rerun-if-changed={linker_script}");
-    println!("cargo:rustc-link-arg=-T{linker_script}");
+    println!("cargo:rustc-link-arg=-Tmultiboot2_chainloader/{linker_script}");
 }

integration-test/bins/multiboot2_payload/build.rs

@@ -1,5 +1,5 @@
 fn main() {
-    let linker_script = "multiboot2_payload/link.ld";
+    let linker_script = "link.ld";
     println!("cargo:rerun-if-changed={linker_script}");
-    println!("cargo:rustc-link-arg=-T{linker_script}");
+    println!("cargo:rustc-link-arg=-Tmultiboot2_payload/{linker_script}");
 }