Bluetooth: hci_raw: Add support for specifying buffer headroom

The HCI transport implemented by an application using the HCI raw
interface may have its own buffer headroom requirements. Currently the
available headroom gets completely determined by the selected HCI
driver. E.g. most of the time this is the native controller driver
which doesn't reserve any headroom at all.

To cover for the needs of HCI raw users, add a new Kconfig variable
for the apps to set to whatever they need. Correspondingly, use the
maximum of the HCI driver and HCI raw headroom requirements for the
buffer pool definitions and the headroom initializations.

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