efl: optional 'upower' ecore system module.

efl: optional 'upower' ecore system module.

Ecore will reach 'upower' using D-Bus system bus in order to detect if
the system state changes and let applications know about the power
state such as low battery or AC power in order to optimize their power
consumption.

For host this is not needed and would not work, since output/host DBus
declares its own output/host/var/run/dbus/system_bus_socket, which has
no dbus-daemon and thus no services in it.

For target it's optional and only installed if BR2_PACKAGE_UPOWER=y,
otherwise it prints error messages about missing upower service.

Signed-off-by: Gustavo Sverzut Barbieri <barbieri@profusion.mobi>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Acked-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>