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>
(cherry picked from commit 87ab610959d9055ff7d28fc6cd7e8f33bd60a1e8)
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/33501
Reviewed-by: He Gao <hegao@google.com>