Andreas, Frederick, thank you for your reply.
We should fix the script.

diff -u -r1.1 walnut-mkimg.sh
--- walnut-mkimg.sh	9 Dec 2002 12:15:50 -0000	1.1
+++ walnut-mkimg.sh	25 Mar 2004 15:07:16 -0000
@@ -19,15 +19,16 @@
 ${OBJCOPY} -O binary ${kernel} ${kernel}.bin.$$
 size=`/bin/ls -l ${kernel}.bin.$$ | awk '{ printf "%d", ( $5 + 511 ) / 512 }'`
 
+# printf "\x00\x52\x50\x4f" > ${output}
+printf "\000\122\120\117" > ${output}
 printf "%d\n%d\n0\n%d\n0\n0\n0\n" $start $size $start |
-    awk 'BEGIN { printf "\x00\x52\x50\x4f" }
-	{
+    awk '{
 		printf "%c", $0 / 256 / 256 / 256 ;
 		printf "%c", $0 / 256 / 256 ;
 		printf "%c", $0 / 256 ;
 		printf "%c", $0 ;
 	}
-    ' > ${output}
+    ' >> ${output}
 
 cat ${kernel}.bin.$$ >> ${output}