image_signing: strip_boot_from_image.sh: convert to info/error helpers

This makes the output easier to follow when multiple scripts are being run.

BRANCH=None
BUG=chromium:714598
TEST=signing images still works

Change-Id: I48edde260e1d1db88f65624c7ff46ad2ac1cc2f4
Reviewed-on: https://chromium-review.googlesource.com/498100
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>

scripts/image_signing/strip_boot_from_image.sh

@@ -30,7 +30,7 @@ fi
 zero_free_space() {
   local rootfs="$1"
 
-  echo "Zeroing freespace in ${rootfs}"
+  info "Zeroing freespace in ${rootfs}"
   # dd is a silly thing and will produce a "No space left on device" message
   # that cannot be turned off and is confusing to unsuspecting victims.
   ( sudo dd if=/dev/zero of="${rootfs}/filler" bs=4096 conv=fdatasync \
@@ -47,7 +47,7 @@ strip_boot() {
   mount_image_partition ${image} 3 ${rootfs_dir}
 
   sudo rm -rf "${rootfs_dir}/boot" &&
-    echo "/boot directory was removed."
+    info "/boot directory was removed."
 
   # To prevent the files we just removed from the FS from remaining as non-
   # zero trash blocks that bloat payload sizes, need to zero them. This was