The goal was to reduce common setup of messages. Changes:
* MailBox turned into singleton to follow existing patterns
* Removed device specific messages from MailBox requiring
clients to know the details instead
* Created base Message class which clients should deriver from
It really simplify the usage for more complicated message queues
like framebuffer setup - see followup commits.
As a demo, query the firmware version. `Meta/serenity.sh gdb aarch64`
can be used to observe that qemu puts 0x548E1 in x0 in response
to this mailbox message.
