ci: build.sh: Join MESON_ARGS and MESON_OPTS

ci: build.sh: Join MESON_ARGS and MESON_OPTS

It is quite confusing seeing these two in a call like this one:
    $ meson build $MESON_OPTS $MESON_ARGS

One has to ask 'how are they different' and 'shouldn't these be
merged'. In fact, these variables hold very different things and we
should make it more obvious. The problem is that renaming MESON_OPTS to
something more meaningful, like 'MESON_CROSS_OPTS' which is what
MESON_OPTS really does would require changes to lcitool and would
impact Dockerfile generation which in turn might have an impact on
other projects which rely on this lcitool functionality which is risky.

Instead, provide a docstring for the former to supplement the latter
and join the two variables in a single one MESON_ARGS which is then
passed to meson's command line so it's a little less confusing.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>