Contains:
    - Bash script generating rrd database and generating graphs from it
    - Python script readding from MQTT and updating rrd database
    - Settings for the Python script
    - Python script regenerating certificates within grace period of their expiry.

DEPENDENCIES:
    - configparser
    - paho
    - rrdtool
    kernel support ( amongst others (: ):
        - CONFIG_USB_VIDEO_CLASS

Steps to get going:
    - Generate rrd file with ./generate_rrd_db.sh -c -i <rrd-file>
    - Set up MQTT broker.
    - Set up Raspberry Pi pico (Pico project in Micropython section)
    - Fill in settings.cfg with your data
    - Set up certificates (Check out Pico project for Bash script generating them)
    - Run Python script.
    - Set up cron job regenerating graphs with frequency to your liking.
    - Set up SSL/TLS certificates with scripts provided. Certificate directory structure shall adhere to the schema below.

(So that I don't forget:)
What is still missing:
    - Python script should run as a daemon.
    - Python script could generate more graphs based on averages from rrd database showing long term trends.
    - Graph generation could be done with rrdcgi (not sure it is needed though)
    - Documentation is lacking detailed step by step setup guide.
    - In order to prevent a rougue client from publishing to a certificate renewal channel, Access Controll List needs to be implemented.
    - There should be a script generating ACL file.
    - Before adding a certificate for a user ACL file shall be updated.

Certificate directory structure:

$ tree -d /etc/mosquitto/certs
├── CA
├── DH
├── clients
│   ├── onge
│   └── pico
├── csr_files
└── server

The acl file shall be located at:
$ /etc/mosquitto/mosquitto.acl

An entry shall be present in the acl file:

$ pattern readwrite cert_reneval/%c/#