[POWERPC][XEN] Mark heap memory based on boot_of.c's allocator.

[POWERPC][XEN] Mark heap memory based on boot_of.c's allocator.
- Explain why we have another allocator (that wasn't so hard now was it?).
- Create and export boot_of_mem_avail() to allow later code to iterate over the
  allocator bitmap.
- Use boot_of_mem_avail() to place memory in the heap, instead of using globals
  and making assumptions about the ordering of reserved areas.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : %5B%CB%15%5E%5D%E5%7C%02%EF%3B8%D4%98%96%25%FF%AF%E3%FC%BE