Allow OS maintainer to use their own video mode settings. Fixes #259

[samnazarko]

This should resolve #259.

• This lets the OS maintainer define "managevideo": "1" in the os.json file if they wish to handle the video modes.
• If this value is not set, or the value is 0, the old behaviour will be unchanged and the 'videomode' from the QVariantMap will be used to populate the config.txt accordingly.

[samnazarko]

@lurch Have you had a chance to look at this yet?

[lurch]

Nope, been too busy with other stuff, sorry.

[NicoHood]

I've updated the patch and merged it into my personal nicohood branch. https://github.com/raspberrypi/noobs/commit/59d14960dabcf75ea786c5df5c0373e558b0fcfd

Ive tested it and it works fine. I really like the idea, since noobs always gets in the way of my monitor settings.

Do not merge this PR, it requires some more changes that I made in my commit. Please refer to the commit above instead.

[samnazarko]

I can update my original commit to include your changes.

[NicoHood]

Please do so. I really like to see this one merged.

[samnazarko]

I have updated my PR against master.

[NicoHood]

You forgot to declare bool _managevideo;. Have a look at my commit again ;)

[samnazarko]

Updated now. Did this in a hurry so have not compile tested.