Bluetooth: Create start_background_scan helper

This patch creates the start_background_scan() function so it can
be reused in the next patch.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Bluetooth: Temporarily stop background scanning on connection

Some LE controllers don't support scanning and initiating a connection
at the same time. So, for those controllers, we should temporarily
stop the background scanning and start it again once the connection
attempt is finished (successfully or not).

So this patch introduces the hci_check_background_scan() which checks
if the background scanning should be started.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>