kernel-install: add header to generate entry files

kernel-install: add header to generate entry files

I was looking at a bug in bugzilla about some boot loader issue, and it was
hard to say if the boot entry files were generated by our plugin or something
else. Add a header to make this clear.

kernel-install invokes the plugins via absolute path always, so $0 gives as
the full path the location where the plugin is installed. This is what we want:

  title      Fedora Linux 37 (Workstation Edition)
  # Boot Loader Specification type#1 entry
  # File created by /usr/lib/kernel/install.d/90-loaderentry.install (systemd 252-409-g5028904^)