Revert "get rid of MemoryHeapPmem"

This reverts commit 7a7524810a0950c1c2b18014a8f1c637275165c4 and
makes MemoryHeapPmem conditioned to BOARD_NEEDS_MEMORYHEAPPMEM

Change-Id: I2f38340984ba87d6970d17ec944e3faded4132c8