Fork vboot_reference in manifest

We need to fork vboot_reference to carry custom patches for signing
behavior.

BUG=b/170128847
TEST=repo init and repo sync
RELEASE_NOTE=None

Change-Id: I7afe7f22b9426364c931cd0f76eb4918849a7fd6
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/15493
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Dexter Rivera <riverade@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/21892
diff --git a/external_full.xml b/external_full.xml
index de2fd43..2009197 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -36,9 +36,6 @@
   <project path="src/scripts"
            name="mirrors/cros/chromiumos/platform/crosutils"
            revision="9d89a314c4d3007e6595999a7cbd106e791df152" />
-  <project path="src/platform/vboot_reference"
-           name="mirrors/cros/chromiumos/platform/vboot_reference"
-           revision="0e97e25e85f0499e23b09a31a2c7116759f191d5" />
   <project path="src/third_party/autotest/files"
            name="mirrors/cros/chromiumos/third_party/autotest"
            revision="4b5b686c5d1892910d6826bd11338837fd9c39cf" />
@@ -78,6 +75,9 @@
   <project path="src/third_party/chromiumos-overlay"
            sync-c="true"
            name="third_party/overlays/chromiumos-overlay" />
+  <project path="src/platform/vboot_reference"
+           name="third_party/platform/vboot_reference"
+           revision="refs/heads/release-R81-12871.B" />
   <project path="src/repohooks"
            name="cos/repohooks"
            revision="refs/heads/master" />