Do not allow publishing artifacts from build stage
Do not allow publishing artifacts from build stage

So that it can only be done after acceptance testing. Added also a
default "false" value for the variable: it is not important but makes it
consistent with the rest.

Signed-off-by: Lluis Campos <9244cb41c39addfc5cf236920b776ea2820513d0@northern.tech>

mendersoftware/meta-mender-qemu