Short: use framebuffer mirror for the boot console.

Short: use framebuffer mirror for the boot console.
Long: the memory region reserved for VideoCore has to be MMU mapped as a
device type memory with disabled caches. Enabling caches will create
hardly predictable artefacts on the screen. Without cache this memory
would be very slow for reading as well as for writing. However, rpi2 has
fast write buffers, which speed up writing to video memory. In summary,
writing to VC ram is much faster than reading form it. Framebuffer
mirror will drastically improve performance when text has to be
scrolled. :)

git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@50314 fb15a70f-31f2-0310-bbcc-cdcc74a49acc