libbt-hci: include bdroid_buildcfg.h from device repo

This lets device repos override some constants with their bdroid_buildcfg.h if
necessary.

This fixes Bluetooth issues (namely tethering) on endeavoru where
BTHC_USERIAL_READ_MEM_SIZE needs to be set to a higher value.

Change-Id: I8955963cdb2b34f1ba63f8160861699f7284814f
Support USB HCI

Issue: AXIA-1459
Change-Id: Ie4cc5766446774a0bae3bbf7d9baa5f44e814f59
Signed-off-by: Ravindranath Doddi <ravindranathx.doddi@intel.com>
Signed-off-by: Krishnan V <krishnanx.vaidyanathan.venkitakrishnan@intel.com>
Signed-off-by: Daniel Leung <daniel.leung@intel.com>
Signed-off-by: Matt Gumbel <matthew.k.gumbel@intel.com>