tree-wide: use the term "initrd" at most places we so far used "initramfs"

tree-wide: use the term "initrd" at most places we so far used "initramfs"

In most cases we refernced the concept as "initrd". Let's convert most
remaining uses of "initramfs" to "initrd" too, to stay internally
consistent.

This leaves "initramfs" only where it's relevant to explain historical
concepts or where "initramfs" is part of the API (i.e. in
/run/initramfs).

Follow-up for: b66a6e1a5838b874b789820c090dd6850cf10513