disk_layout_util.sh: Pass -I flag to mkfs.fat

This was to be compatible with the upgrade of the portage package
sys-fs/dosfstools from 3.0.27 to 4.1.
This ignores a *possibly* wrong error message about formatting
a device which is already partitioned. See crbug.com/901479 for
more discussion.

BUG=chromium:901479
TEST=building seems to work as before, the error we are covering
   up seems to be a problem with udev, possibly because we're
   inside chroot.
CQ-DEPEND=CL:1323910

Change-Id: I4d53b05036a285eb3b54af84f1486cc3bb532cf1
Reviewed-on: https://chromium-review.googlesource.com/1323474
Commit-Ready: Nick Crews <ncrews@chromium.org>
Tested-by: Nick Crews <ncrews@chromium.org>
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/build_library/disk_layout_util.sh b/build_library/disk_layout_util.sh
index 6f62967..be9f9ff 100644
--- a/build_library/disk_layout_util.sh
+++ b/build_library/disk_layout_util.sh
@@ -444,7 +444,9 @@
         "${fs_options_arr[@]}"
     ;;
   fat|vfat)
-    sudo mkfs.vfat -n "${fs_label}" "${part_dev}" "${fs_options_arr[@]}"
+    # -I flag is needed to ignore a (we think) false error about formatting
+    # a device that already has partitions on it
+    sudo mkfs.vfat -I -n "${fs_label}" "${part_dev}" "${fs_options_arr[@]}"
     ;;
   squashfs)
     # Creates an empty squashfs filesystem so unsquashfs works.