package/matchbox: make match-startup-monitor a real package

Currently, matchbox-startup-monitor is missing a Kconfig entry, but is
so far solely a build-time dependency of matchbox-panel.

So, make it a real package, and only build-depend on it when enabled.
Fix the condition to build-depend on it.

To keep the existing behaviour, default to 'y' when startup-notification
is enabled.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>