Bug#370644: #370644 update-grub exits with non-zero exit status

[Florian Weimer]

* Goswin von Brederlow:

> could you please give some examples where it breaks and ay why it is
> grubs fault that it gets called with the wrong number of arguments.

Note that "it" = write_kernel_entry.

The bug is triggered on a system where memtest86 is installed and
/boot is on separate partition.  Any non-Linux OS will probably
trigger it, too.

For details, run

  grep -2 write_kernel_entry /sbin/update-grub 

and count the arguments.  Two invocations have been changed, but
another two haven't.