Add and exchange global heap configuration CONFIG_HEAP_MEM_POOL_SIZE
against configuration used for auto heap calculation
CONFIG_HEAP_MEM_POOL_ADD_SIZE_ZBUS.
Add CONFIG_HEAP_MEM_POOL_ADD_SIZE_ZBUS as related configuration
Clarify that HEAP is used for zbus in a whole
Consistent naming 'Zbus'/'zbus' to 'ZBus'
Signed-off-by: Matthias Plöger <matthias.ploeger@phoenixcontact.com>