core: add callout service

Add a callout service to call registered callback functions at a given
time from now. A callout is periodic or oneshot depending on how the
callback function returns. Callb

core: add callout service

Add a callout service to call registered callback functions at a given
time from now. A callout is periodic or oneshot depending on how the
callback function returns. Callback functions execute in nexus and
interrupt context.

The callout service is initialized with callout_service_init() and
depends on an abstract timer interface to manage the hardware timer.
callout_service_cb() needs to be called from the timer interrupt handler
to drive the callout service.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>

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