This allows us to keep track of the extra menu entries specified in the
OS configuration.
* gnu/system.scm (<boot-parameters>)[bootloader-menu-entries]: New field.
(read-boot-parameters): Initialize it.
(operating-system-boot-parameters): Likewise.
(operating-system-boot-parameters-file): Serialize it.
* gnu/bootloader.scm (menu-entry->sexp, sexp->menu-entry): New
procedures.