Fix syslinux failure on new USB drive

tools/build_usb_boot.sh

@@ -21,22 +21,25 @@ if [ -b $DEST_DIR ]; then
     # We have a block device, install syslinux and mount it
     DEST_DEV=$DEST_DIR
     DEST_DIR=`mktemp -d mntXXXXXX`
+    mount $DEST_DEV $DEST_DIR
+
+    if [ ! -d $DEST_DIR/syslinux ]; then
+        mkdir -p $DEST_DIR/syslinux
+    fi
 
     # Install syslinux on the device
     syslinux --install --directory syslinux $DEST_DEV
-
-    mount $DEST_DEV $DEST_DIR
 else
     # We have a directory (for sanity checking output)
-	DEST_DEV=""
+    DEST_DEV=""
-	if [ ! -d $DEST_DIR/syslinux ]; then
+    if [ ! -d $DEST_DIR/syslinux ]; then
-	    mkdir -p $DEST_DIR/syslinux
+        mkdir -p $DEST_DIR/syslinux
-	fi
+    fi
 fi
 
 # Get some more stuff from syslinux
 for i in memdisk menu.c32; do
-	cp -p /usr/lib/syslinux/$i $DEST_DIR/syslinux
+    cp -p /usr/lib/syslinux/$i $DEST_DIR/syslinux
 done
 
 CFG=$DEST_DIR/syslinux/syslinux.cfg