ax25: Switch to CONFIG_BOOTP_PREFER_SERVERIP

ax25: Switch to CONFIG_BOOTP_PREFER_SERVERIP

The ax25-ae350 target currently uses CONFIG_BOOTP_SERVERIP which means we
ignore the DHCP provided TFTP ip address. This breaks every case where we
do now provide a serverip environment variable.

Instead, let's use the new CONFIG_BOOT_PREFER_SERVERIP option to fall back
to the DHCP provided TFTP IP if no serverip environment variable is set.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Rick Chen <rick@andestech.com>