Merge pull request #992 from bitcraze/tobbeanton/kbuild_vs_pesistent_…

…param

Updated platform.md with KBuild vs persistent param.

docs/userguides/platform.md

@@ -26,10 +26,18 @@ Each platform supports one or more device types. The device type is identified i
 to fit the hardware. The Crazyflie 2.0 and Crazyflie 2.1 are for instance two device types handled by the `cf2` platform,
 they will configure the firmware to use different drivers for the IMU based on the device type.
 
+## Persistent paramters
+While KBuild is ment for configuring the compile time specific functionality selections the 
+[persistent paramters](/docs/userguides/logparam.md#persistent-parameters) are there to configure it in run time. 
+And this is the rule of thumb. If it is about a configuration, such as PID tuning or low voltage level, it should 
+be a persistent parameter. If it is a functionality it should be a compile time KBuild define. It is not a sharp cut, 
+do what is best and easiest for the user.
+
 ## Default values
 
 There is functionality to set default values for variables based on the platform and this is for instance used to set
-different default tuning values for the (small) Crazyflie 2.1 and the (larger) Crazyflie Bolt.
+different default tuning values for the (small) Crazyflie 2.1 and the (larger) Crazyflie Bolt. The default values are 
+located in e.g. `src/platform/interface/platform_default_cf2.h` for the Crazyflie 2.1.
 
 ## Implementation