kbuild: re-enable Makefile generation in a new O=... directory

The commit:
18c32dac "kbuild: fix
building with O=.. options"
disabled the creation of a Makefile in a new O=... directory. Restore it.

Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>