Add LOCALVERSION so we can append strings that show up in uname
without having to fiddle with the Makefile and EXTRAVERSION, etc.

* localversion* files are read first
* config variable is appended last
* LOCALVERSION from the command line overrides all of this
* check is forced on build, since we can't really know when
  the config or environment options change.
Signed-off-by: Ian Wienand <ianw@gelato.unsw.edu.au>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>