Fix memory erasure when booted with systemd

systemd handles kexec in a different way than sysvinit, so we have to adapt our stuff a bit:

• https://bugs.debian.org/699008
• https://bugs.debian.org/747514
• https://wiki.archlinux.org/index.php/kexec
• Right way to do kexec (see the rest of the thread too)

Feature Branch: feature/jessie

Parent Task: #5821 (closed)

Related issues

• Related to #7468 (closed)
• Has duplicate #8099 (closed)

Original created by @intrigeri on 7183 (Redmine)