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] 6e432d5ecb46959f3c27dbbca48e41512bb40a70
[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>

boot/syslinux/Config.in

@@ -35,6 +35,7 @@ config BR2_TARGET_SYSLINUX_PXELINUX
 
 config BR2_TARGET_SYSLINUX_MBR
 	bool "install mbr"
+	depends on !BR2_TOOLCHAIN_HAS_BINUTILS_BUG_19615
 	select BR2_TARGET_SYSLINUX_LEGACY_BIOS
 	help
 	  Install the legacy-BIOS 'mbr' image, to boot off a