tools/setlocalversion: 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.,
1.3.2-rc3).  This makes it much easier to recognize "newer" versions
and to see how much has been changed since the referenced tag.

Stolen from Linux kernel's scripts/setlocalversio, see commit d882421f.

Signed-off-by: Wolfgang Denk <wd@denx.de>