Add firmware updater examples to CI

CI was not building the a.rs application due to the requirement of b.bin
having been built first. Add a feature flag to examples so that CI can
build them including a dummy application.

Update a.rs application examples so that they compile again.

examples/boot/application/rp/Cargo.toml

@@ -29,6 +29,7 @@ debug = [
     "embassy-boot-rp/defmt",
     "panic-probe"
 ]
+skip-include = []
 
 [profile.release]
 debug = true