[srv.logger] Create new logger service

Split the functionality of outputting kernel logs out of the UART
driver, and into a new service. The UART driver now registers a console
out channel with the service locator, which the logger service
retrieves, and then enters a loop getting logs from the kernel and
printing them out to the console.

assets/manifests/default.yaml

@@ -4,4 +4,5 @@ programs:
   - name: panic.serial
     target: kernel
     flags: panic
+  - name: srv.logger
   - name: drv.uart