Upgrade to syslinux 6.03-pre18 or later

syslinux 6.03-pre1 has bugs (e.g. #7232 (closed)). We want to upgrade it.

Feature Branch: bugfix/7173-upgrade-syslinux

Subtasks

• #7174 (closed)
• #7683 (closed)

Related issues

• Related to #6578 (closed)
• Blocks #6576 (closed)
• Blocks #7232 (closed)
• Blocks #7231 (closed)
• Blocks #6397

Original created by @intrigeri on 7173 (Redmine)