qemu-build: Update base image to ubuntu:bionic
qemu-build: Update base image to ubuntu:bionic

qemu 5.2 fails to build under Ubuntu 16.04. Upgrade to the base image
to the one specified by scripts/build-qemu-robot-docker.sh (bionic,
18.04).

Change-Id: Id135f0c3118a6aa997ddcd578fe98af4a9ae17de
Signed-off-by: Andrew Jeffery <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@aj.id.au>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts