as syslinux >5 was masked due to #FL-288
and this issue was fixed in 5.01

and in changelog it states:
EXTLINUX: Fix installation and subdirectory patching.
ldlinux: Fix multiple buffer overflows in cmdline parsing code that resulted in files failing to run and cmdlines being truncated.

and this should be done since commit 129a5845aec4d6c750c4bddd936f315fb441d2fa

=> unmask syslinux 5.01