image_signing: sign_official_build.sh: use loopbacks for dump_config

This avoids copying out the kernels just to read their configs.
Not super important as the signer doesn't use it, but we want to
kill off the grab_kernel_config helper.

BRANCH=None
BUG=chromium:714598
TEST=dump_config still works

Change-Id: I2533b1d4de6980120f277fea3a1d964cb4fbaf0d
Reviewed-on: https://chromium-review.googlesource.com/505479
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>

scripts/image_signing/sign_official_build.sh

@@ -859,9 +859,10 @@ sign_image_file() {
 case ${TYPE} in
 dump_config)
   check_argc $# 2
+  loopdev=$(loopback_partscan "${INPUT_IMAGE}")
   for partnum in 2 4; do
     info "kernel config in partition number ${partnum}:"
-    grab_kernel_config "${INPUT_IMAGE}" ${partnum}
+    sudo dump_kernel_config "${loopdev}p${partnum}"
     echo
   done
   exit 0