uboot: Honor static network settings

If the user has manually configured all required network parameters,
honor that setup, by skipping DHCP, and directly fetch the image.

board/common/uboot/scripts/ixprepdhcp.sh

@@ -1,6 +1,6 @@
 setenv autoload no
 
-if dhcp; then
+if test -n "${ipaddr}" -a -n "${netmask}" -a -n "${serverip}" -a -n "${bootfile}" || dhcp; then
     setenv proto tftp
     setenv dltool tftpboot