systemd secure boot support in git submodule

Cloning

When cloning this repo, please use git clone --recurse-submodules <URI>.

Without this, the solus secure boot certificate will not be included in the build manifest after a successful build.

Updating

If you already have a clone of the systemd repo but have not yet added the git submodule, you can do so with:

git submodule update --init --recursive

This will initialise and fetch an up-to-date version of the submodule into your working copy.