net: lwip: dhcp: Save DHCP siaddr field to tftpserverip env var

net: lwip: dhcp: Save DHCP siaddr field to tftpserverip env var

The DHCP siaddr field contains the IP address of next server to use in
bootstrap. Typically this will be the IP address of a TFTP server or the
IP address of the DHCP server itself.

RFC 2131, 2. Protocol Summary, Page 10:

   DHCP clarifies the interpretation of the 'siaddr' field as the
   address of the server to use in the next step of the client's
   bootstrap process.  A DHCP server may return its own address in the
   'siaddr' field, if the server is prepared to supply the next
   bootstrap service (e.g., delivery of an operating system executable
   image).  A DHCP server always returns its own address in the 'server
   identifier' option.

Set the 'tftpserverip' env variable when the siaddr field contains an
IP address that is different compared to the DHCP server IP address.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Jerome Forissier <jerome.forissier@arm.com>