nrf: call sd_app_evt_wait when the SoftDevice is enabled

This reduces current consumption from 500-1000µA to very low (<10µA)
current consumption. This change is important for battery powered
devices, especially devices that may be running for long periods of
time.

src/runtime/runtime_stm32.go

@@ -2,6 +2,8 @@
 
 package runtime
 
+import "device/arm"
+
 type timeUnit int64
 
 func postinit() {}
@@ -12,3 +14,7 @@ func main() {
 	run()
 	abort()
 }
+
+func waitForEvents() {
+	arm.Asm("wfe")
+}