Bluetooth: btrtl: Add support for a config filename postfix

The contents of the rtl_bt/rtlXXXX_config.bin file may be board specific
allow the caller of btrtl_initialize to specify a postfix identifying
the board, which if specified will make btrtl_initialize look for
rtl_bt/rtlXXXX_config-<postfix>.bin instead.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>