Document Common use cases for using specific functions

[lanefu]

Extend this https://docs.armbian.com/Developer-Guide_Build-Preparation/#executing-any-bash-statement

to document common use cases for testing such as

• building bsp packages only
• building Armbian-config only
• checking out source and patching only

[EvilOlaf]

Also explain that this is nothing where users can input values and the build system does whatever they are asking for. Directly at least.