support/testing: test_lxc: use ARM arm external toolchain

support/testing: test_lxc: use ARM arm external toolchain

Until now, the lxc test was using the ARM CodeSourcery 2014.05 armv5 toolchain.
But the recent systemd version bump to 245 added a toolchain dependency
on systemd package due to build issues with gcc < 5.0.

Before [1] the lxc test was failing to build with the ARM CodeSourcery 2014.05
toolchain. After [1], the test is faling at runtime since the
"BR2_INIT_SYSTEMD=y" symbol disapear from the dot config (.config) due to
the new toolchain dependency.

Fix this by using the same toolchain as for the systemd tests [2]

[1] 2196ee25ff29c87001a42a382655d5b52d5de3c4
[2] b3d979c0d1b10de501576644e7dcdba708889b0c

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>