re-enable trainer/student for i6x

[marook]

re-enables trainer/student for the i6x target. only supports connection via jack cable. tested with two FlySky I6X RCs.

[ajjjjjjjj]

Hi, thanks for contribution!
Since this project is very tight on space and this is mostly the same code as for external module PPM, common code should be taken to common functions and used in both external and trainer PPM mode.
This way binary size will not grow too much.