Makefile: Use file name instead of file descriptor for .config target

[AlfioEmanueleFresta]

The project makefile uses fd 9 to redirect output, when generating the contents of the .config file. Previously, it used fd 10, but was changed due to compatibility concerns (c88913e).

The build fails in environments where this file descriptor can't be used. I've encountered this error trying to build tinycbor as part of a dependency, from a Cargo crate build script which invokes make (see logs):

make[2]: *** read jobs pipe: Bad file descriptor.  Stop.
make[2]: *** Waiting for unfinished jobs....
/bin/sh: 1: 9: Bad file descriptor

Proposing a fix, changing the OUT variable to take a path to append to, instead of a redirection.

I've tested the change by running make, and verifying the contents of .config are the same as before my change.

[thiagomacieira]

Thanks!