Mega-patch for OpenFirmware loader support.

• Flesh out ofw_readin routine.
• Add OpenFirmware load and exec routines.
• Make sure memory allocation for the kernel is done correctly.
• Change the way the heap is allocated so as to make it easier to deallocate when we hand over.
• Add a command to print memory maps similar to the one for ia64.

With this patch, I can now load and hand over to a kernel on my iMac. There
are some problems with OpenFirmware routines failing after the hand over that
still need to be addressed.