Support multi-volume snapshot boot

[bmr-cymru]

Support to boot multiple snapshot volumes (e.g. /var, /home etc.) has been requested by numerous users and is a requirement to use boom to support OS upgrade rollbacks for non-trivial file system layouts.

To enable this a component is required to allow overriding /etc/fstab during boot.

An RFE has been submitted to the systemd project, systemd/systemd#27260. If this is unsuccessful then boom will need to either ship this component itself or depend on some new component that provides the required functionality.

[bmr-cymru]

Resolved in commit ed0a12a.