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/nrf/Cargo.toml

@@ -24,3 +24,4 @@ cortex-m-rt = "0.7.0"
 [features]
 ed25519-dalek = ["embassy-boot/ed25519-dalek"]
 ed25519-salty = ["embassy-boot/ed25519-salty"]
+skip-include = []