toolchain: enable paranoid unsafe path check by default

This check is enabled for all build in the Buildroot autobuilders since [1]
(i.e since 2015.02 release).

Enable this option by default.

[1] https://git.busybox.net/buildroot-test/commit/?id=41552e2a249de4568aa01f14bd588d1c8166f70b



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