Bluetooth: Add support for writting characterisitc value

This adds write and flush attribute callbacks to be used for writting
attribute values.
With this patch lower tester can test if IUT can properly handle
write request to the characteristic values.

> ACL Data RX: Handle 64 flags 0x02 dlen 7  [hci0] 1151923.440823
      ATT: Read Request (0x0a) len 2
        Handle: 0x0003
< ACL Data TX: Handle 64 flags 0x00 dlen 7  [hci0] 1151923.456050
      ATT: Read Response (0x0b) len 2
        Value: bbbb
> ACL Data RX: Handle 64 flags 0x02 dlen 9  [hci0] 1151928.435525
      ATT: Write Request (0x12) len 4
        Handle: 0x0003
          Data: ffff
< ACL Data TX: Handle 64 flags 0x00 dlen 5  [hci0] 1151928.456865
      ATT: Write Response (0x13) len 0
> ACL Data RX: Handle 64 flags 0x02 dlen 7  [hci0] 1151929.380796
      ATT: Read Request (0x0a) len 2
        Handle: 0x0003
< ACL Data TX: Handle 64 flags 0x00 dlen 7  [hci0] 1151929.394636
      ATT: Read Response (0x0b) len 2
        Value: ffff

Change-Id: Iea1d39e1a52496692141411251fd4ac2f9ae0b97
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>