Insert a script into systemd boot order at a specific location

["Christoph Pleger" <Christoph.Pleger@cs.tu-dortmund.de>]

Hello,

for many years and up to Debian 7, I am using update-rc.d to define a
specific location in the boot order, where a script is executed which
automatically performs some administrative tasks. The script is executed
after only very basic tasks have been performed, and before all other
tasks. The definition of this boot order is especially easy with the LSB
init script format, because using  Required-Start und X-Start-Before in my
init script are sufficient.

My question now is, how can I achieve almost the same boot order in Debian
8 with systemd? To summarize, the boot order should be like this:

1. Only very basic things, like starting the static network connection and
mounting file systems

2. My script, with no other systemd boot task running concurrently

3. Everything else

Regards
  Christoph