[stm32f4-discovery] Using WFI instead of nop in the main loop.

WFI (Wait for Interrupt) tells the processor to suspend untill the next
interrupt is called. Better than burning away the cycles with nop.

examples/stm32/f4/stm32f4-discovery/timer/timer.c

@@ -24,6 +24,8 @@
 #include <libopencm3/cm3/nvic.h>
 #include <libopencm3/stm32/exti.h>
 
+#include <libopencmsis/core_cm3.h>
+
 uint16_t frequency_sequence[18] = {
 	1000,
 	500,
@@ -183,8 +185,16 @@ int main(void)
 	gpio_setup();
 	tim_setup();
 
+	/* Loop calling Wait For Interrupt. In older pre cortex ARM this is
+	 * just equivalent to nop. On cortex it puts the cpu to sleep until
+	 * one of the three occurs:
+	 *
+	 * a non-masked interrupt occurs and is taken
+	 * an interrupt masked by PRIMASK becomes pending
+	 * a Debug Entry request
+	 */
 	while (1)
-		__asm("nop");
+		__WFI(); /* Wait For Interrupt. */
 
 	return 0;
 }