Fix I2C documentation

[agners]

Proposed change

There is no support of updating config.txt via CONFIG USB flash drive.
Document how to find the hassos-boot partition to configure config.txt
directly on it and use the hassos-boot config method.

Also, instead of using i2c1 use the preferred (according to RPi
documenation) name i2c_vc.

Adresses home-assistant/operating-system#825

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase:
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: Enable I2C on the Home Assistant Operating System via import from usb does not work operating-system#825

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[frenck]

Thanks, @agners 👍