Add default kernel rescue initrd boot option

[ermo]

The goal is to have our boot menus (which are managed by clr-boot-manager) show a default boot option and a rescue initrd option via the addition of rd.break=cleanup to the kernel command line.

This is particularly useful for Solus, because kernels and initrds are built and distributed together, which means that since we can test that the interactive rescue shell in the initrd works for us, there is a very high likelyhood that it will also work on user systems for recovery purposes (resetting passwords etc.)

[ermo]

@silkeh Any objections to keeping this in 4.6 and doing it as part of the grub-2.12 testing in #1348 ?

[silkeh]

I'd like to keep this separate from Grub seeing as it also includes systemd-boot. No objections on keeping it in 4.6 if people don't mind it landing somewhere in week 21.