Otherwise we cannot overwrite them in mulitconfig files. It's not
expected that anything else in the layer config files sets those
variable, thus shadows our setting in local.conf.
Something changed in latest bitbake to reveal this issue. We first see
it in Isar since bitbake moved to f8126aaf7741.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>