tools: add an option to generate cscope database

When --cscope option is given to Scons, it will generate a cscope
cross-reference database in current directory, which is useful in
Vim(and other cscope-aware text editors). For example, `scons -s
--cscope` will do nothing except generating the database. You can use
this option with other options together.

It is inspired by the `make cscope` of Linux.
Merge pull request #110 from grissiom/cscope

tools: add an option to generate cscope database