client: Add advertise.discoverable command

client: Add advertise.discoverable command

This adds advertise.discoverable command which can be used to set it
own instance:

[bluetooth]# advertise.discoverable on
[bluetooth]# advertise on

@ MGMT Command: Add Advertising (0x003e) plen 14
        Instance: 1
        Flags: 0x00000001
          Switch into Connectable mode
        Duration: 0
        Timeout: 0
        Advertising data length: 3
        Flags: 0x02
          LE General Discoverable Mode
        Scan response length: 0
< HCI Command: LE Set Advertising Data (0x08|0x0008) plen 32
        Length: 3
        Flags: 0x02
          LE General Discoverable Mode

Change-Id: I664dce815fd1d6122177c4b135ecaf89f17270ff
Signed-off-by: Amit Purwar <amit.purwar@samsung.com>