Support DHCP Option 61

The DHCP client of the lwIP stack SHALL be able to send the DHCP Option 61 (Client Identifier) with all DHCP messages.
The DHCP option 61 is defined in RFC 2132 (section 9.14. Client-identifier).

The Client-Identifier consist of a type-value pair:

    Code   Len   Type  Client-Identifier
   +-----+-----+-----+-----+-----+---
   |  61 |  n  |  t1 |  i1 |  i2 | ...
   +-----+-----+-----+-----+-----+---
n SHALL be 1 + length of Client Identifier. 
n (max) = 48
t1 SHALL be 0 (according to RFC2132)

The lwIP interface SHALL be extended so that a Client Identifier can be configured into the DHCP client.

It SHALL be possible to dynamically activate/deactivate the sending of Option 61 within the DHCP client.