[ppm] check some rssi level to have a valid ppm frame

Default is always true
It is required on some receivers that are still produicing valid frames
after RC lost (futaba R617FS). It is still possible to get the lost
flag from an rssi reading or at least with a GPIO (usually possible when
the receiver has a led to indicate the RC lost state).
[actuators/asctec] disable the i2c hack for the general case

It can be activated for lisa/m boards with the flag:
USE_I2C_ACTUATORS_REBOOT_HACK

This issue should be handled at the i2c level (#71)
[actuators/asctec] fix path and add support for motor configuation to v2