Makefile: fix make <blah>_defconfig

Make sure we only pass O=<path> if we were originally called like that,
otherwise we might end up looking for the .config in the wrong place.

Likewise, when changing between using O= or not (or simply running
make <blah>_defconfig in a pristine source tree), we do end up having
a .config, but no .config.cmd. It isn't really critical so just continue
even if we couldn't read it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>