support/scripts/boot-qemu-image.py: wait before using expect

support/scripts/boot-qemu-image.py: wait before using expect

As reported by a gitlab runtime test [1] and on the mailing list
[2], some runtime tests are failing on slow host machines when
the qemu-system-<arch> is missing on the host.

The boot-qemu-image.py script need to wait some time after
calling pexpect.spawn() in order to make sure that the qemu
process has been executed in start-qemu.sh.

If start-qemu.sh failed due to missing qemu-system binary
an exception will be thrown by child.expect() and should be
catched by the error handling (pexpect.EOF).

After spending a lot of time to investigate with Yann E. MORIN
[3]. It seems that short-lived child processes are a corner-case
that is not very correctly handled...

Without adding a sleep(1), child.expect() can trigger an
exception before setting the exitstatus of the spawned
process. This issue can be reproduced on a gitlab runner or
by adding "exit 1" in the first line of start-qemu.sh
(after the shebang).

There is even the same workaround in some pexpect examples [4].

Thanks to Yann for the help while investigating the issue.

Tested:
https://gitlab.com/kubu93/buildroot/pipelines/138472925

[1] https://gitlab.com/kubu93/buildroot/pipelines/135487475
[2] http://lists.busybox.net/pipermail/buildroot/2020-April/280037.html
[3] http://patchwork.ozlabs.org/project/buildroot/patch/20200418161023.1221799-1-romain.naour@gmail.com/
[4] https://github.com/pexpect/pexpect/blob/master/examples/ssh_tunnel.py#L80

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/509053135

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: reorder imports]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>