license: Replace Apache boilerplate with SPDX tag

Replace the existing Apache 2.0 boilerplate header with an SPDX tag
throughout the zephyr code tree. This patch was generated via a
script run over the master branch.

Also updated doc/porting/application.rst that had a dependency on
line numbers in a literal include.

Manually updated subsys/logging/sys_log.c that had a malformed
header in the original file.  Also cleanup several cases that already
had a SPDX tag and we either got a duplicate or missed updating.

Jira: ZEP-1457

Change-Id: I6131a1d4ee0e58f5b938300c2d2fc77d2e69572c
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
diff --git a/misc/ring_buffer.c b/misc/ring_buffer.c
index 21cdab6..395ddd4 100644
--- a/misc/ring_buffer.c
+++ b/misc/ring_buffer.c
@@ -3,17 +3,7 @@
 /*
  * Copyright (c) 2015 Intel Corporation
  *
- * 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.
+ * SPDX-License-Identifier: Apache-2.0
  */
 
 #include <misc/ring_buffer.h>