bin/fk: add option -u to force usbboot, even if SSH is/seems available

bin/fk

@@ -1,4 +1,7 @@
 #!/bin/sh -e
+if [ "$1" = -u ]; then
+    NN=
+else
     if [ "${1#usb}" = "$1" ]; then
 	if ! ping -c 1 -w 1 "$1" >/dev/null; then
 	    echo "no ping response from $1" 1>&2
@@ -9,6 +12,7 @@ else
 	NN=`ifconfig ${1:-usb0} |
 	  sed '/.*inet addr:\([^ ]*\).*/{s//\1/;s/100$/101/;s/200$/202/;p;};d'`
     fi
+fi
 if [ "$NN" ]; then
     echo "SCP to $NN" 1>&2
     scp uImage $NN: