Bluetooth: Remove rand driver and use entropy instead

After porting the rand driver to drivers/entropy, replace the usage of
the old, Buetooth-specific driver with the generic entropy one.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>