Bluetooth: Add basic discovery commands to the management interface

This patch adds start_discovery and stop_discovery commands to the
management interface. Right now their implementation is fairly
simplistic and the parameters are fixed to what user space has
defaulted to so far.
This is the very initial phase for discovery implementation into
the kernel. Next steps include name resolution, LE scanning and
bdaddr type handling.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Anderson Briglia <anderson.briglia@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>