Makefile: generate wrapper makefile when running make without a .config

Makefile: generate wrapper makefile when running make without a .config

The recent change to error out instead of running menuconfig when no .config
is available broke an existing use case:

make O=output-foo; cd output-foo; br-init-conf (or similar to get a sensible .config); make

As there is no wrapper makefile in output-foo.

Fix it by ensuring the wrapper gets created if needed.

Signed-off-by: Peter Korsgaard <[email protected]>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>
Signed-off-by: Peter Korsgaard <[email protected]>