boot/syslinux: disable syslinux legacy-BIOS for broken toolchains

Since [1] syslinux is built with the target toolchain in order to
properly build with gnu-efi package. But toolchains built with
binutils 2.26 break the syslinux legacy-BIOS build as reported at [2],
due to binutils bug #19615.

Thanks to Benoît Allard for the investigation and the link to the
binutils bug [3].

[1] 6e432d5e
[2] http://lists.busybox.net/pipermail/buildroot/2017-July/196253.html
[3] https://sourceware.org/bugzilla/show_bug.cgi?id=19615



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Benoît Allard <benoit.allard@greenbone.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>