enable pmw3901 flow if (new) parameter SENS_EN_PMW_FLOW is true

[BazookaJoe1900]

Describe problem solved by the proposed pull request
the startup scripts try to start the pmw3901 flow on some boards by default. even if the drone doesn't has one connected.
that cause false errors on startup script for drones that doesn't has the sensors (most of the configurations)

ERROR [pmw3901] driver start failed

Describe your preferred solution
New SENS_EN_PMW_FLOW parameter that allows to define if the sensor should be or not on the system.

Describe possible alternatives
With this parameter, starting the flow can moved to common rc.sensors file. curently its only on boards that compile the pmw3901.
In that case, boards that don't have the driver compiled will show "Parameter SENS_EN_PMW_FLOW not found" (that is being delt on #12832)
does that prefered?

[BazookaJoe1900]

Another thing, On AV/ax-1 I didn't add the code, because there is strange 'if' that if there is no flow there is adis16497. Who can address what to do there? I don't know the reason for current implementation
https://github.com/PX4/Firmware/blob/43fdcd787697f7d1d9e611268e59edc635908650/boards/av/x-v1/init/rc.board_sensors#L16-L20

[dagar]

How about SENS_EN_PMW3901 and move it more centrally? Boards that don't support it won't even have the parameter (it would be pruned).

[BazookaJoe1900]

@dagar done.
small quistion, the compretion tell me there is no EOL in the end of some scripts. Just want to make sure that its ok.

[LorenzMeier]

@BazookaJoe1900 No, that's not ok - please add the newlines.

[BazookaJoe1900]

Ok, I will fix that later.
What about ax1? Currently there is double call to start the driver.

[dagar]

Ok, I will fix that later.
What about ax1? Currently there is double call to start the driver.

For av-x check the parameter, but only to start the ADIS16977 if the PMW3901 is disabled.