uboot: Support overriding the interface used for netbooting Infix

For systems with multiple paths to the network, different situations
may call for different paths to be used. Therefore, allow the user to
override the active interface via the environment on the auxiliary
partition.

The default (defined the bootloader's device tree) is still used in
cases when nothing else is specified in the environment.

board/common/uboot/scripts/ixpreboot.sh

@@ -33,7 +33,7 @@ for tgt in "${boot_targets}"; do
 	setexpr ixmenu_n ${ixmenu_n} + 1
 
 	if load ${devtype} ${devnum}:${auxpart} ${loadaddr} /uboot.env; then
-	    env import -b ${loadaddr} ${filesize} BOOT_ORDER DEBUG
+	    env import -b ${loadaddr} ${filesize} BOOT_ORDER DEBUG ethact
 	fi
 
 	if test -n "${DEBUG}"; then