docs: document how to change the default password

Add a document showing how to change the default password.

Signed-off-by: Marta Rybczynska marta.rybczynska@linaro.org

docs/default-password.rst

+.. SPDX-FileCopyrightText: Linaro Ltd
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+.. include:: definitions.rst
+
+.. _DefaultPassword:
+
+Setting the Default Password
+############################
+
+|main_project_name| provides a default user with default password.
+Device vendor should change this password for their devices and disable the remote
+login if not needed.
+
+The default password is configured by the variable **"ONIRO_USER_PASSWORD"**.
+You can define in in your ``local.conf`` using a command like (replace the password
+and the seed by your own values):
+
+.. code-block:: console
+
+    mkpasswd -m sha-512 <yourpassword> -s "seed"
+
+Replace the seed and password with your own data, also make sure to escape all special shell
+characters, the `%q` formating of shell `printf` is useful. The final command could be:
+
+For example:
+
+.. code-block:: console
+
+    $ printf "%q" $(mkpasswd -m sha512crypt 123456 -s "seed1234" )
+    \$6\$seed1234\$qd8wosDovm0k4OSCKGSbnwrj4SJf.92UyAqEgdE3xhD0tHxhqqoSxqmsjiXfj4ue0ih4txcKHQWqGQ.1HZmm7.
+
+In this case, your ``local.conf`` (or an bbappend in one of your layers) should contain:
+
+.. code-block:: console
+
+    ONIRO_USER_PASSWORD="\$6\$seed1234\$qd8wosDovm0k4OSCKGSbnwrj4SJf.92UyAqEgdE3xhD0tHxhqqoSxqmsjiXfj4ue0ih4txcKHQWqGQ.1HZmm7."
+
+After rebuilding your image, you should be able to login with the `123456` password.
+
+.. Note::
+
+   The password and seed in this document is shown for illustration purposed only. In a production
+   sytem, use different values than the default ones, or what has been shown in this document.