Proper sleep implementation in Pycardium

The current sleep in Pycardium (based on mxc_delay()) is not ideal for a number of reasons:

1. It busy-spins for the duration of the delay which wastes energy unnecessarily and does not allow for any sort of sleep.
2. It does not run scheduled interrupt handlers. This is most noticeable with button-interrupts as implemented in !297.
3. It is not interruptible by CTRL-C.

The best solution to this is probably a re-implementation of the logic behind mxc_delay(). This means defining a SysTick_Handler to count overflows and a delay-function which checks the timer value on every IRQ wakeup. In (simplified) code:

void systick_delay(uint32_t usec)
{
	/* Setup SysTick */
	/* ... */

	while (waiting) {
		/* Handle scheduled interrupt handlers/callbacks */
		mp_handle_pending();
		/* Wait for any IRQ, including the SysTick */
		__WFI();
	}

	/* Tear down SysTick */
	/* ... */
}

Note that this implementation does not deal with CTRL-C correctly yet (point 3). I'm not quite sure what is needed for that to work properly.