In gitlab, there is no extra CI job running for commits in a merge
request (MR). This means, we can't run different code in a MR against
"master".
So instead of only checking whether all devices are booting if there's
a MR against master, always perform this check.
I've edited the message slightly, so it's clear that it's only required
to have the devices booting when making a merge request against master.
This uses a variable configured in the gitlab project [settings->CI/CD->Variables]
to restrict when the qemu job will run. This should prevent the job from running
on repo forks where an appropriate runner has not been configured/registered.
Note that if this variable is set in a repo where there is no runner registered to
run qemu, then any CI jobs in that repo will ultimately fail the qemu tests since
no runner will be found.