git-svn (and git) doesn't handle empty directories, so add .empty files
to those dirs like elsewhere in buildroot.
Those empty directories are normally not a big deal, but the recent changes
to u-boot broke the build.
Mark empty directories in target skeletons with .empty files
and remove them while copying to target (like it's already done for
svn and cvs files) to better support version control system which don't
handle empty directories (E.G. git-svn).
