Makefile cleanup

[nirajnsharma]

• Combine link and link_d targets in Makefile. Make link_d have a dependency on link, and the default target should be link_d
• Move PositCore Makefile to top-level and rename to Makefile.system. Migrate paths appropriately.
• Create separate RTL generation targets which do not include the TBs

[nirajnsharma]

@riyarjain I have transferred this one to me as I'd rather you look at the compile errors and the BDPI stitchup.