Makefile: allow to build tags and cscope

On behalf of Nishanth:

It is handy to have cscope and tags built from the sources to
account for any change in source without having to run special
scripts
So, add option to 'make tags' and 'make cscope' to generate
the references.

While at it, ignore these generated files from git tracking.

Signed-off-by: Nishanth Menon <nm@ti.com>