kbuild: fix first module build

When building a specific module before doing a total kernel
build it failed because $(MORVERDIR) were missing.
Creating the MODVERDIR explicit (independent of KBUILD_MODULES)
fixed this. As a side-effect the MODVERDIR will be created
also for a non-module build - but no harm done by that.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>