From: Sam Ravnborg <sam@ravnborg.org>

The external module support recently introduced caused a number of problems:
- To build an external module the Module.symvers file was needed
- To create the Module.symvers file a module was required
- If Module.symvers was missing kbuild boiled out with an error
- If vmlinux was missing also the stage 2 of module build failed (make -k)
- It was not documented what was needed to actually bauild a module

The following patch addresses this by adding the following functionality:
- Always generate the Module.symvers file
- Ignore a missing Module.symvers file
- Add a new target modules_prepare, it prepares the kernel for building
  external modules, and is also usefull with O=
- And it adds some more comments to Makefile.modpost, so others may follow
  it with some luck
- .modpost.cmd is no longer generated

This should close all reports on issues with respect to building external
modules with current kernel - which has been identified as kernel problems.