client: Add advertise.discoverable-timeout command

client: Add advertise.discoverable-timeout command

This adds advertise.discoverable-timeout command which can be used to
limit the amount of time the advertisement is discoverable:

[bluetooth]# advertise.discoverable on
[bluetooth]# advertise.discoverable-timeout 10
[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
@ MGMT Event: Advertising Added (0x0023) plen 1
        Instance: 1
@ MGMT Event: Command Complete (0x0001) plen 4
      Add Advertising (0x003e) plen 1
        Status: Success (0x00)
        Instance: 1
@ MGMT Command: Add Advertising (0x003e) plen 14
        Instance: 1
        Flags: 0x00000001
          Switch into Connectable mode
        Duration: 0
        Timeout: 0
        Advertising data length: 3
        Flags: 0x00
        Scan response length: 0

Change-Id: I4d4a936d529355d5ae0092b84e7e62d0c174bf61
Signed-off-by: Amit Purwar <amit.purwar@samsung.com>