This is a general fix for properly initializing private copperplate
heaps managed by the heapmem allocator:

- the heapmem descriptor must be allocated separately, not to consume
  space from the heap memory. At this chance, heapmem_init() must be
  given the right descriptor address, not NULL.

- heaps must account for the size of the meta data the allocator
  needs. Make sure to allocate this extra-space when no specific heap
  memory has been given.

- upon array allocation, align the item size used in calculating the
  overall heap size on the next power-of-two boundary, so that the
  required number of items can be obtained from the pool (as the
  caller may rightfully assume).

Without these changes, the private copperplate allocator simply cannot
work on top of heapmem, so keep them grouped.
Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>