Add timestamp of next non-boot event to DBUS signal next_bootup_event

DSME needs to know two things about events:

1) The trigger time of the next event that requires the device
to be booted, DSME programs the RTC WKALM accordingly.

2) The trigger time of the next event which does not require
the device to be booted, DSME uses this information to calculate
when the device can be suspended.

(1) is already covered by the implementation of the DBUS signal
next_bootup_event. This change set adds support for (2): it
introduces a second parameter to next_bootup_event, which is a
timestamp of the next alarm that does not require that the device
is powered up, case (2).