[build] Ensure that generated filesystem images retain no stale content

We use mformat to ensure that the FAT filesystem starts as empty.
However, formatting the filesystem can still leave old data blocks
present (though unreferenced) within the disk image.

Truncate the image to a zero length before extending, to ensure that
no stale content is retained.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/genfsimg

@@ -312,7 +312,6 @@ fi
 if [ -n "${FATIMG}" ] ; then
     FATSIZE=$(du -s -k "${FATDIR}" | cut -f1)
     FATSIZE=$(( FATSIZE + PAD + 256 ))
-    touch "${FATIMG}"
     if [ "${FATSIZE}" -le "1440" ] ; then
 	FATSIZE=1440
 	FATARGS="-f 1440"
@@ -325,6 +324,8 @@ if [ -n "${FATIMG}" ] ; then
 	FATSERIAL=$(( SOURCE_DATE_EPOCH % 100000000 ))
 	FATARGS="${FATARGS} -N ${FATSERIAL}"
     fi
+    touch "${FATIMG}"
+    truncate -s 0 "${FATIMG}"
     truncate -s "${FATSIZE}K" "${FATIMG}"
     mformat -v iPXE -i "${FATIMG}" ${FATARGS} ::
     mcopy -i "${FATIMG}" -s "${FATDIR}"/* ::