Fixes <https://bugs.gnu.org/36876>.
Reported by Jesse Gibbons <jgibbons2357@gmail.com>.
Previously 'guix system delete-generations' or 'switch-generation' would
lose the extra bootloader menu entries specified in the current system's
configuration. This fixes that.
* guix/scripts/system.scm (reinstall-bootloader): Turn PARAMS into a
single <boot-parameters>. Adjust ENTRIES to include extra menu entries
specified in PARAMS.