Install syslinux.exe in the ISO filesystem on Buster

Since syslinux 3:6.03+dfsg1-1 this file is (rightfully) not included anymore in the Debian source package so we cannot copy it from there anymore if we keep relying on UUI using our own syslinux.exe.

Related issues

Related to #15292 (closed)

Original created by @intrigeri on 15178 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information