tools/mkconstfs: Add an improved tool.

The new tool (mkconstfs2) features:

* more robust filename handling: no need for mangling,
  and works on Windows.
* Better output generation: nothing is written in case
  of failures.
* Allows more control over the files that are included:
 - does not traverse directories, filenames must be explicitly
   given.
 - The "root" can be explicitly given (thus the tool can get
   the same result independently of the CWD).

Thanks to MichelRottleuthner for making it work with Windows paths.