Move implementation of hci_driver.h exposed functions to a common file.
The functions exposed by hci_driver.h header file is implemented in
either hci_core.c, or hci_raw.c. But since these functions would be
identical for both implementations have them in a new file that is
included for both.
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
