TARGET_NRF5 - Add critical section enter/exit overrides for NRF5 targets.

This change takes advantage of the reworked primitives of SDK v11.

hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/nordic_critical.c

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2015-2016, ARM Limited, All Rights Reserved
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <stdint.h>                  // uint32_t, UINT32_MAX
+#include <assert.h>                  // uint32_t, UINT32_MAX
+#include "cmsis.h"
+#include "nrf_soc.h"
+#include "nrf_sdm.h"
+#include "nrf_nvic.h"
+
+static volatile uint8_t  _sd_state = 0;
+static volatile uint32_t _entry_count = 0;
+
+void core_util_critical_section_enter()
+{
+    // if a critical section has already been entered, just update the counter
+    if (_entry_count) {
+        ++_entry_count;
+        return;
+    }
+
+    // in this path, a critical section has never been entered
+    // routine of SD V11 work even if the softdevice is not active
+    sd_nvic_critical_region_enter(&_sd_state);
+
+    assert(_entry_count == 0); // entry count should always be equal to 0 at this point
+    ++_entry_count;
+}
+
+void core_util_critical_section_exit()
+{
+    assert(_entry_count > 0);
+    --_entry_count;
+
+    // If their is other segments which have entered the critical section, just leave
+    if (_entry_count) {
+        return;
+    }
+
+    // This is the last segment of the critical section, state should be restored as before entering
+    // the critical section
+    sd_nvic_critical_region_exit(_sd_state);
+}