support/testing: TestLxc switch to the Arm Bootlin toolchain

TestLxc uses systemd as init but the recent update to systemd v254
requires a toolchain w/ linux headers >= 4.14 to provide
LOOP_SET_BLOCK_SIZE [1] (added in systemd v253 [2]).

Since no other toolchain that the Bootlin one is available
switch to it.
(ARM Arm toolchain requires BR2_ARM_CPU_HAS_NEON enabled)

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=89e4fdecb51cf5535867026274bc97de9480ade5
[2] https://github.com/systemd/systemd/commit/1163ddb386ef46f63942171e6eab0ca64eb818e4

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4768561390

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/testing/tests/package/test_lxc.py

@@ -10,6 +10,7 @@ class TestLxc(infra.basetest.BRTest):
             BR2_cortex_a9=y
             BR2_ARM_ENABLE_VFP=y
             BR2_TOOLCHAIN_EXTERNAL=y
+            BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
             BR2_LINUX_KERNEL=y
             BR2_LINUX_KERNEL_CUSTOM_VERSION=y
             BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.15.38"