drivers: bluetooth: build as static library

Instead of putting object files inside libzephyr.a,
simply build a separate static library as most other
driver types are doing this already.

Note that the include to subsys/bluetooth has been
moved from under drivers/bluetooth to subsys/bluetooth,
as it is actually the subsystem's job to make sure
the include directories are correct.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>