Inject certificate used by sign_update function into mixer's chroot

[gtkramer]

A file named os-core-update-extra-files needs to be created in mixer's server state directory containing the paths that need to be added to the os-core-update manifest. These files need to be created with the proper permissions inside mixer's full chroot. This needs done after mixer build bundles but before mixer build update. Without this, users are not able to perform swupd update from the content that mixer creates when custom update signing is used.

At a minimum, need the following:

/usr/share/clear/update-ca
/usr/share/clear/update-ca/Swupd_Root.pem

This is because swupd-client has an option in it's configure.ac for:

default_cert_path="/usr/share/clear/update-ca/Swupd_Root.pem"

[mbelluzzo]

#88 was merged. So I think this can be closed now, right?