config.json: Add monitoring-agent to default image configuration

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build.mk: remove a few unintended lines from make progress

Tell "make" not to print two silly comments to its output.
Also, the "call quiet" macro prints an empty line if you don't give
it anything to print, so when we have nothing good to print, let's just
use "@" instead, to avoid the empty lines.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>