Use groups rather than separate manifests

repo allows projects to be placed into groups. Use this feature, rather
than separate manually-maintained manifests, to allow syncing the flasher
source excluding the tools repositories.

Also add a group for the U-Boot repo, since it may be useful to exclude
that if you're building U-Boot elsewhere, and manually copying its build
results into the flasher directory. This change would have multiplied
the number of manifests by two if not implemented using repo's group
feature.

Signed-off-by: Stephen Warren <swarren@nvidia.com>