Update license heading for all source files.

[jeandudey]

We currently use the Apache-2.0 license, and it requires to have on each source file a reference to the license in use, in our case Apache-2.0.

/**
 * Copyright 2020 btcven and Locha Mesh developers
 *
 * 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.
 */

/**
 * @{
 *
 * @file
 * @brief         Dictatorship mandatory code
 *
 * @author        Nicolás Maduro <eldictadorprograma@presidencia.com.ve>
 * @}
 */

This is an example of how each file should have their license and the respective documentation comment containing the @file which should be blank, author and a brief description of what the file does. Ideally the author is the one who created the code so when someone modifies it one can know immediately who wrote it and who to ask in case of the need to understand something.

[luisan00]

linked with PR #157

[jeandudey]

This is fixed now.