kbuild: remove unused -r option for module-init-tool depmod

Following a thread on busybox mailing list
depmod -r option is ignored by module-init-tools depmod
-r option break busybox depmod.

So the best solution look to remove -r from kernel Makefile

Signed-off-by: Gilles Espinasse <g.esp@free.fr>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>