kbuild: modules_install for external modules must not remove existing modules

kbuild: modules_install for external modules must not remove existing modules

When installing external modules with `make modules_install', the
first thing that happens is a rm -rf of the target directory. This
works only once, and breaks when installing more than one (set of)
external module(s).
With following fix we have the functionality:
- for a in-kernel modules_install the $(MODLIB)/kernel directory will be
  deleted before module installation
- for external modules the existing modules will be left as is assuming
  one may be building and installign several external modules

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>