Add a "verify" option to sign_official_build.sh.

scripts/image_signing/common.sh

Index: scripts/image_signing/common.sh
diff --git a/scripts/image_signing/common.sh b/scripts/image_signing/common.sh
index ffc344cb22c4f140168b1139cf0e72df3e1cabfd..37834dc541493dfedb067e514a46af9208a1ee8b 100755
--- a/scripts/image_signing/common.sh
+++ b/scripts/image_signing/common.sh
@@ -45,9 +45,9 @@ extract_image_partition() {
   local output_file=$3
   local offset=$(partoffset "$image" "$partnum")
   local size=$(partsize "$image" "$partnum")
-  dd if=$image of=$output_file bs=512 skip=$offset count=$size conv=notrunc
+  dd if=$image of=$output_file bs=512 skip=$offset count=$size conv=notrunc >/dev/null 2>&1
 }
-  
+
 # Replace a partition in an image from file
 # Args: IMAGE PARTNUM INPUTFILE
 replace_image_partition() {