busybox

Makefile: fix cscope target

This target doesn't work with current directory layout. Just make cscope
index all .c and .h files.

While we're at it: add cscope output files to .gitignore.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>