Comment 8 for bug 1733603

Any update on this?

As I workaround I used nest repositories
https://help.launchpad.net/Packaging/SourceBuilds/Recipes#Nesting

It wasn't so easy to do, because all this subrepos should be imported to LP manually:
1. open you account
2. open Code tab
3. On the right Configure Code
4. Import a Git repository hosted somewhere else: here put an url to github repo and name the same as submodule.
5. Press Update
6. After importing a new LP repository will be created but it will reuse existing name. We'll fix that in 8 step.
7. Do the same for other submodules if any.
8. Fix back main code repo: "Link to a Git repository already on Launchpad" and select an old repo name.
9. Edit recipe and for each submodule create a nest record.

Here is an example recipe:

# git-build-recipe format 0.4 deb-version {debupstream}-{time}
lp:openwrt master
nest libubox lp:~stokito/openwrt/+git/openwrt-libubox libubox
nest uci lp:~stokito/openwrt/+git/openwrt-uci uci
nest ubus lp:~stokito/openwrt/+git/openwrt-ubus ubus
nest ustream-ssl lp:~stokito/openwrt/+git/openwrt-ustream-ssl ustream-ssl
nest uhttpd lp:~stokito/openwrt/+git/openwrt-uhttpd uhttpd
nest rpcd lp:~stokito/openwrt/+git/openwrt-rpcd rpcd
nest jsonpath lp:~stokito/openwrt/+git/openwrt-jsonpath jsonpath
nest mountd lp:~stokito/openwrt/+git/openwrt-mountd mountd