This ensures graphical progress is shown ASAP when dependencies are
equally likely to run.
Otherwise, when the graphical step was ready, and switchroot about to
run, the shell step could run before the splash. This means that, in
practice, a long running task like e2fsck could have prevented the
splash to show. It doesn't make sense to make the other tasks depend on
a splash task!
There is no intention to implement any other special-cased ordering. If
there is the need to, it might mean that there is a deficiency in the
design.
Instead, rely on targets.
Additionally removes the "Boot" special dependency which only made sense
when it was expected that having no dependency was an error.
