client: Add scan.discoverable command

client: Add scan.discoverable command

This adds discoverable command to scan menu which can be used to set
if adapter should become discoverable while scanning:

[bluetooth]# scan.discoverable on
[bluetooth]# scan on
SetDiscoveryFilter success
[CHG] Controller XX:XX:XX:XX:XX:XX Discoverable: yes
Discovery started
[CHG] Controller XX:XX:XX:XX:XX:XX Discovering: yes
[bluetooth]# scan off
Discovery stopped
[CHG] Controller XX:XX:XX:XX:XX:XX Discoverable: no

Change-Id: Ica34e2bc17c72460b5fd41e8104cff2bb5fa0234
Signed-off-by: himanshu <h.himanshu@samsung.com>