kbuild: change CONFIG_LOCALVERSION_AUTO to use a git-describe-ish format

Change the automatic local version to have the form -nnnnn-gSHA1SUMID,
where 'nnnnn' is the number of commits since the last tag (i.e.,
2.6.21-rc7).  This makes it much more likely that the package names created
for the kernel will look "newer" to a package manager.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>