OpenTx default channel center

[rguptaca]

What is the history/reason behind:
- using 1500us and 512us as default values?
- not providing a way to change these defaults?

My servo centerer has 1520us as it's center. I use this while building or adjusting my models. When I use my Teranis I use companion to change all the center values to 1520us.

[Kilrah]

Nearly all radios used to use 1500us for center, only Futaba was 1520.
You can change center separately for each servo, which you'd ideally do when setting up a model.