support/testing/test_syslinux: add missing Kconfig symbol

BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y is needed to use the
custom external toolchain x86-i686--glibc--bleeding-edge-2018.11-1.tar.bz2

Otherwise the symbol BR2_TOOLCHAIN_EXTERNAL_URL is lost.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981734
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981733

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit ddf40cffeab1552d350a07c26129c9e83239399c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/testing/tests/boot/test_syslinux.py

@@ -6,6 +6,7 @@ class TestSysLinuxBase(infra.basetest.BRTest):
         """
         BR2_x86_i686=y
         BR2_TOOLCHAIN_EXTERNAL=y
+        BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
         BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
         BR2_TOOLCHAIN_EXTERNAL_URL="http://toolchains.bootlin.com/downloads/releases/toolchains/x86-i686/tarballs/x86-i686--glibc--bleeding-edge-2018.11-1.tar.bz2"
         BR2_TOOLCHAIN_EXTERNAL_GCC_8=y