[LEDE-DEV] [RFC] build: adjust version number handling

[Hannu Nyman]

Hi Jow,

I tested your patch in my build and it seemd to work ok for the current 
master branch. Also LuCI showed version info sensibly.

But you might clarify a bit the config option explanation texts. Especially 
VERSION_CODE could be hard to understand If the reader does not know the 
intended use in advance. (maybe add an example of a possible value and of the 
typical default value.) The explanation also contains references to svn and 
git-svn, which may look odd in a pure-git build environment. Could the last 
sentence be something like "it defaults to the commit count since last 
branching/tag of the build tree", or something like that. (whatever is true 
with the new logic)