- Redo mailbox stuff

- Redo mailbox stuff
- Idea is to create mailbox.library and compile it as kernel object and link it with kernel-kobj and exec-kobj into bootstrap
- _ReadMailbox and _WriteMailbox should then be callable from bootstrap and once multitasking is up library calls wrap around them with semaphore or atomic protection
- Rasperry Pi port can't at this moment live without the mailbox stuff so mailbox support is needed at the bootstrap, this just reduces code size as the same code is used in the library
- Not much free time but I'll continue this as soon as I can. No function has the correct parameters so this is just a stub framework

git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@46350 fb15a70f-31f2-0310-bbcc-cdcc74a49acc