signer scripts: use make_temp_dir helper

The common code takes care of cleaning up temporary files for us if we
use the right helpers.  These scripts don't though, so the temp files
end up not getting cleaned and over time, /tmp/ files up with crap.

The common helper takes care of unmounting and removing, so converting
these files over should be safe.

BUG=None
TEST=`./signer/signer_poller.py -s` signs local images and leaves no /tmp/tmp.* junk

Change-Id: I87aa122895997a5ec0017665203fdc8d14e0ab2a
Reviewed-on: https://gerrit.chromium.org/gerrit/20329
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>

scripts/image_signing/remove_test_label.sh

@@ -13,7 +13,6 @@
 set -e
 image=$1
 
-rootfs=$(mktemp -d)
+rootfs=$(make_temp_dir)
 mount_image_partition ${image} 3 ${rootfs}
-trap "umount ${rootfs}; rm -rf ${rootfs}" EXIT
 sed -i 's/test//' "${rootfs}/etc/lsb-release"

scripts/image_signing/set_lsb_release.sh

@@ -47,9 +47,8 @@ EOF
     exit 1
   fi
 
-  local rootfs=$(mktemp -d)
+  local rootfs=$(make_temp_dir)
   mount_image_partition_ro "$image" 3 "$rootfs"
-  trap "sudo umount $rootfs; rm -rf $rootfs" EXIT
   if [ -n "$key" ]; then
     sudo umount "$rootfs"
     mount_image_partition "$image" 3 "$rootfs"