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>

configs/ax25-ae350_defconfig

@@ -40,3 +40,4 @@ CONFIG_DM_SPI=y
 CONFIG_ATCSPI200_SPI=y
 CONFIG_TIMER=y
 CONFIG_ATCPIT100_TIMER=y
+CONFIG_BOOTP_PREFER_SERVERIP=y

include/configs/ax25-ae350.h

@@ -11,7 +11,6 @@
  * CPU and Board Configuration Options
  */
 #define CONFIG_BOOTP_SEND_HOSTNAME
-#define CONFIG_BOOTP_SERVERIP
 
 /*
  * Miscellaneous configurable options