tests/prom-env: Ease time-out problems on slow hosts

Peter Maydell recently ran into time-out problems with the
prom-env test on a rather slow ARM board. To tackle this issue,
we can speed up the test by running QEMU with "-nodefaults" for
the pseries machine, so that SLOF has less devices to scan during
boot, and by using the "nvramrc" environment variable instead of
"boot-command", since this variable is evaluated earlier in the
boot process.
And to be really sure that we do not face such time out problems
again, let's also increase the time out value from 100s to 120s
instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1486739699-1076-1-git-send-email-thuth@redhat.com
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>