Rebooting on the Dell E5420 often hangs with the keyboard or ACPI
methods, but is reliable via the PCI method.
[ hpa: this was deferred because we believed for a long time that the
recent reshuffling of the boot priorities in commit dfd6b7fa997f1e5a524451dab944f164fe99cda1 fixed this platform.
Unfortunately that turned out to be incorrect. ]