BUGFIX: new pwm behaviour of tinkerboard

[gen2thomas]

• fix #818 path issue similar to implementation in beaglebone adaptor. Introduce sysfs.Find() to avoid func in struct.
• fix #818 function order problems in PWMPin().
• fix #818 naming problem with "inverted" by introducing sysfs.SetPolarity()
• read period to use in ServoWrite() - TODO resolved.
• implement SetPeriod() to ensure automatic adjustment of duty cycle.
• expand error messages in sysfs.pwm_pin.go for better trouble shooting, some other improvements
• adjust tests

[gen2thomas]

Revert to draft because the name for inverted polarity must be "inversed" instead of "inverse"

[gen2thomas]

@deadprogram ready to review & merge

[deadprogram]

Thank you very much for working on this @gen2thomas looks good! Now merging.