manifest: add ledge forks

Add LEDGE forks with kirkstone-related changes. We use forks because
the meta-ts layer doesn't have kirkstone support yet,
and meta-ledge-secure has some dependencies we do not need right now
(especially meta-selinux). Discussions for the two layers are in
progress, but may still take some time. When all our required changes
are upstream, we may switch to using the upstream directly.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>