kbuild: a better way to generate cscope database change

It's a problem about cscope target of kernel Makefile, and the cscope
plugin of emacs:
1. `make cscope` will generate cscope.files cscope.{in,po,}.out;
2. the cscope plugin expect a cscope.out.{in,po,};
3. the default `cscope -b` would generate cscope.{in,po,}.out;

There are three approach to solve it:
1. modify the cscope C code;
2. modify the cscope emacs plugin lisp code;
3. modify the Makefile;

I have tried to communicate with the cscope upstream, but later I
realize the third approach is most meaningful.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>