Add u-boot to manifest

Needed to compile the SDK.

BUG=b/177367546
TEST=repo init, repo sync, build sdk parts that need u-boot.
RELEASE_NOTE=None

Change-Id: Ib85ce984a7c679aa4cc655d2b2068db795b12587
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/29930
Reviewed-by: Dexter Rivera <riverade@google.com>
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
diff --git a/external_full.xml b/external_full.xml
index 4c2b9ff..c47de77 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -63,6 +63,9 @@
   <project path="infra_virtualenv"
            name="mirrors/cros/chromiumos/infra_virtualenv"
            revision="cab8a95f2961561eb56a95d6f2bfc685686db75a" />
+  <project path="src/third_party/u-boot/files"
+           name="mirrors/cros/chromiumos/third_party/u-boot"
+           revision="39ce6d27ad29fd324793a8d0c7db8ae712cc027c" />
 
   <!-- COS sources -->
   <project path="src/overlays"