nrf: refactor SoftDevice enabled check

Share the code with the runtime.

src/runtime/runtime_nrf_softdevice.go

@@ -13,17 +13,24 @@ func sd_app_evt_wait()
 // This is a global variable to avoid a heap allocation in waitForEvents.
 var softdeviceEnabled uint8
 
+// Check whether the SoftDevice is currently enabled.
+// This function is also called from the machine package, so the signature has
+// to stay consistent.
+func isSoftDeviceEnabled() bool {
+	// First check whether the SoftDevice is enabled. Unfortunately,
+	// sd_app_evt_wait cannot be called when the SoftDevice is not enabled.
+	arm.SVCall1(0x12, &softdeviceEnabled) // sd_softdevice_is_enabled
+
+	return softdeviceEnabled != 0
+}
+
 func waitForEvents() {
 	// Call into the SoftDevice to sleep. This is necessary here because a
 	// normal wfe will not put the chip in low power mode (it still consumes
 	// 500µA-1mA). It is really needed to call sd_app_evt_wait for low power
 	// consumption.
 
-	// First check whether the SoftDevice is enabled. Unfortunately,
-	// sd_app_evt_wait cannot be called when the SoftDevice is not enabled.
-	arm.SVCall1(0x12, &softdeviceEnabled) // sd_softdevice_is_enabled
-
-	if softdeviceEnabled != 0 {
+	if isSoftDeviceEnabled() {
 		// Now pick the appropriate SVCall number. Hopefully they won't change
 		// in the future with a different SoftDevice version.
 		if nrf.Device == "nrf51" {