Make CW270FLIP default MAG alignment

[DzikuVx]

Current setup for default mag aligment and mag alignment in particular was somehow strange:

• DEFAULT setting was not really a DEFAULT as it was just exposing what MAG chipset returned. The same as CW0
• Per board settings were valid only for boards equipped with internal MAG, which in 2021 is a dead trend. I'm not aware of any modern FC with integrated MAG that is even remotely popular
• 270FLIP is a de-facto standard for all external GPS+MAG modules

This PR makes a 270FLIP a default value is per-board config does not say otherwise. Most probably, it satisfies the needs of 95% of the users. Users with "custom" setups can still manually set MAG orientation