generate_medkits: this is more general script instead of generate_medkit

We need to generate more than one medkit per target. That can be controlled by CI but it is better to have it all at once controlled by turris-build repository. It also partially hides all possible options generate_medkit provides. Most of them are to have versatile tool to generate medkit but are not intended to be used in CI to build real medkits. With this script (named generate_medkits to be consistent with generate_lists) only minimal required info is required from user and rest is provided depending on what kind of medkit is build.

To have repository clean generate_medkit was moved to helpers directory.

There are hints on future variants of medkits. At the moment primary use is for contracts but we can also provide minimal medkits (pretty much compared to what we have), beef up default medkit (just include some more common package lists) and also for example have full medkit with all package lists and drivers and languages (it is question if we even want that). It is also expected to have more targeted medkits such as broadband variant that would out of the box provided support for LTE modems and so on.