Fix duplicate Syslinux entries

Documentation of open()[1] says:

  Other common values ... and 'a' for appending (which on some Unix
  systems, means that all writes append to the end of the file
  regardless of the current seek position).

On my system (Linux 5.4), writes to a file opened as 'a+' go at the end.

[1] https://docs.python.org/3/library/functions.html#open