net: lwip: add TFTPSERVERIP Kconfig option

net: lwip: add TFTPSERVERIP Kconfig option

With the legacy networking stack, it is possible to use USE_SERVERIP,
SERVERIP and BOOTP_PREFER_SERVERIP Kconfg options to force use of a
specific TFTP server ip.

Using the lwIP networking stack use of the 'tftpserverip' environment
variable provide the closest equivalent functionality.

Add USE_TFTPSERVERIP and TFTPSERVERIP Kconfig options that can be used
to add the 'tftpserverip' environment variable to force use of a
specific TFTP server ip.

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