Bluetooth: Export helpers for defining buffer pools

Now that the incoming ACL & HCI event pools can be defined externally
to hci_core.c there should be convenience macros to know what's the
minimum required buffer size for these pools.

Change-Id: I6f2a7322a3d77e1304e9d925767a1fd3471c76c7
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>