scripts/gen_image_generic.sh: make ext4 bootfs reproducible

Set fixed timestamp for kernel other files in /boot filesystem.
This should help making x86 *combined* images reproducible.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 068ea2cde0)

scripts/gen_image_generic.sh

@@ -34,7 +34,7 @@ if [ -n "$GUID" ]; then
     mkfs.fat -n kernel -C "$OUTPUT.kernel" -S 512 "$((KERNELSIZE / 1024))"
     mcopy -s -i "$OUTPUT.kernel" "$KERNELDIR"/* ::/
 else
-    make_ext4fs -J -L kernel -l "$KERNELSIZE" "$OUTPUT.kernel" "$KERNELDIR"
+    make_ext4fs -J -L kernel -l "$KERNELSIZE" ${SOURCE_DATE_EPOCH:+-T ${SOURCE_DATE_EPOCH}} "$OUTPUT.kernel" "$KERNELDIR"
 fi
 dd if="$OUTPUT.kernel" of="$OUTPUT" bs=512 seek="$KERNELOFFSET" conv=notrunc
 rm -f "$OUTPUT.kernel"