[kernel] Replace endpoint with new mailbox API

The new mailbox kernel object API offers asynchronous message-based IPC
for sending data and handles between threads, as opposed to endpoint's
synchronous model.

assets/manifests/default.yaml

@@ -4,6 +4,4 @@ programs:
   - name: panic.serial
     target: kernel
     flags: panic
-  - name: drv.uefi_fb
-    flags: graphical
   - name: drv.uart