kbuild: fix modules_install after a 'make vmlinux'

make vmlinux would delete the content of $(MODVERDIR)
equals .tmp_versions. This caused a subsequent
make modules_install to fail.

Fix it so we clean the directory only for the
modules build - but we still unconditionally create it so
we can do:
make dir/file.ko
without a preceeding make modules.

Reported by David Miller <davem@davemloft.net>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: David Miller <davem@davemloft.net>