net: bootp: as CONFIG_BOOTP_SERVERIP is defined, keep bootfile not changed

Currenly when CONFIG_BOOTP_SERVERIP is defined, the SERVERIP is not changed
when receive the BOOTP packet. But BOOTFILE is changed via BOOTP packet.

As we will load the BOOTFILE from SERVERIP, if the BOOTFILE is modified
by bootp packet but SERVERIP is not, that is not make sense.

This patch make SERVERIP and BOOTFILE be consistent. If we define the
CONFIG_BOOTP_SERVERIP, then SERVERIP and BOOTFILE will not changed by
BOOTP packet. Only IP address is changed.

Signed-off-by: Josh Wu <josh.wu@atmel.com>