Signed-off-by: Stefan Weil <sw@weilnetz.de>

Sometimes it is useful to override the default version detection, for
example when local patches are applied to a repo. Allow providing a
version number using a file called .scmversion, which is the same that
the Linux kernel and U-Boot use.

This summaries giving an overview of a scripts function and a short summary
how it's doing this. Only the scripts are covered, that are used by the
Freifunk-Berlin firmwarebuiler too.

[Matthias Schiffer: slightly reworded some descriptions]