Add new vendor specific attribute 768 in TCP/IP Interface Object that holds the "Client Identifier" used for DHCP Option 61

Add new vendor specific attribute in TCP/IP Interface Object that holds the "Client Identifier" used for DHCP Option 61.

• The new vendor specific attribute 768 shall be added that has the following structure:
   

  Name	Data Type	Description	Default
  Client Identifier	Struct of:
  	USINT	Number of bytes that shall be used for OPTION 61 0: Option 61 deactivated 1-48:  Option 61 activated 49-255: Reserved for future use. Indicates the number of bytes from below ARRAY that will be used for DHCP Option 61	0
  	ARRAY of 48 USINTs	DHCP Option 61 content	All zero

• The attribute itself shall be supported by default. However, if necessary it can be disabled by the host application.
• The attribute shall be stored in remanent data (non volatile).
• The attribute shall be used to configure/activate/deactivate the DHCP Option 61 feature in the lwIP stack (relates to PSLWIP-453).
• Per default, the attribute SHALL be gettable from the network and host application
• Per default, the attribute SHALL be settable from the network and host application