Fix building of examples which do not use defmt

Uses build.rs in board directories to set the linker options locally.
The file .cargo/config can't be used for that, as in a workspace the
local .cargo/config is ignored if the build is triggered from the
workspace root.

.cargo/config

@@ -31,7 +31,6 @@ rustflags = [
     "-C", "link-arg=-Tlink.x",
     "-C", "inline-threshold=5",
     "-C", "no-vectorize-loops",
-    "-C", "link-arg=-Tdefmt.x",
 ]
 
 # This runner will make a UF2 file and then copy it to a mounted RP2040 in USB

boards/pimoroni-plasma-2040/build.rs

@@ -0,0 +1,6 @@
+//! This build script makes sure the linker flag -Tdefmt.x is added
+//! for the examples.
+
+fn main() {
+    println!("cargo:rustc-link-arg-examples=-Tdefmt.x");
+}

boards/pimoroni-tiny2040/build.rs

@@ -0,0 +1,6 @@
+//! This build script makes sure the linker flag -Tdefmt.x is added
+//! for the examples.
+
+fn main() {
+    println!("cargo:rustc-link-arg-examples=-Tdefmt.x");
+}

boards/rp-pico/build.rs

@@ -0,0 +1,6 @@
+//! This build script makes sure the linker flag -Tdefmt.x is added
+//! for the examples.
+
+fn main() {
+    println!("cargo:rustc-link-arg-examples=-Tdefmt.x");
+}