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