install/{keymap,sd-vconsole}: replace "kernel" with "@kernel"
systemd changed the magic string to keep the kernel's default keymap from "kernel" to "@kernel" to avoid namespace issues.
See https://github.com/systemd/systemd/commit/bf77d59772722e96508df91bd24e1b6940b3edfb
Fixes: b0c109f1 ("/install/{keymap,sd-vconsole}: do not fail on KEYMAP=kernel")