Add System / RouterBOARD metrics

[clau-bucur]

Allows to detect current firmware version and whether firmware upgrade is required.

[akpw]

@clau-bucur thanks for the PR, a few comments:

• FE_ROUTERBOARD_KEY = True by default is problematic, both because of general backward-compatibility policy and because of the specific issues it would create for CHR / non-RouterBOARD devices. that would likely turn into noisy per-scrape error logging unless users manually disable the feature
• there should be a configuration template update in mktxp/cli/config/mktxp.conf
• there should be a configuration template update in deploy/kubernetes/secret.yaml
• a dedicated test would be great, feel free to review the existing ones to see if you can add one

[akpw]

finished by 18a6d32