qemuimage-tests/sanity/boot: Increase timeout

As we've increased the parallelisation on the build servers, we've started to see
core-image-minimal sanity test boot failures where the network never comes up. We
don't see those failures for core-image-sato, its always minimal.

Looking at the results, it can take ~100 seconds for the network to come up,
even on the sato images if the machine has a high load. The timeout for the boot
test is only 120 seconds compared to 400 on every other test.

This change makes the timeout equal for all the tests at 400 seconds in the hope
that the load on the autobuilder is causing the sanity tests to run slowly and
hence triggering the false negatives.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>