* kern/powerpc/ieee1275/init.c: Rename HEAP_LIMIT to HEAP_MAX_ADDR,
and make it easier to figure out.
Add HEAP_MIN_SIZE and HEAP_MAX_ADDR definitions.
(grub_claim_heap): Use HEAP_MAX_ADDR rather than taking a parameter.
Do not avoid claiming a region above HEAP_MAX_ADDR if that would
leave us with less than HEAP_MIN_SIZE total heap.
Avoid our total amount of heap to surpass HEAP_MAX_SIZE.
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