Update Trim Like Open Tx

[Sahartman21]

Thank you for supporting upgrades in ethos. I fly models where flight modes 0, 1, and 2 are used within a couple of minutes. It is desirable to have like used for open TX.

Use own trim
Use FM0 trim
Use FM0 plus own trim as offset.

[bsongis-frsky]

One of the 1.5.0 features!