Fix guess-rev.sh where the builtin echo doesn't support -n

git-svn-id: svn://svn.berlios.de/openocd/trunk@1337 b42882b7-edfa-0310-969c-e2dbd0fdcd60

guess-rev.sh

@@ -5,5 +5,5 @@ REV=unknown
 
 which svnversion > /dev/null 2>&1 && REV=`svnversion -n`
 
-echo -n $REV
+`which echo` -n $REV