Makefile update for both core and uvm

This PR updates the e40p Makefiles so that both the core testbench and uvmt environment use similar rules for writting compiler outputs and logs to a single location. I have also included a few other items:

1. Remove unused modules from the uvmt_cv32e40p file-list.
2. Minor edits of READMEs.
3. Renamed cv32e40p/sim/Common.mk to ExternalRepos.mk.

Item 3 is worth thinking about. For each of our cores there are two Common.mk Makefiles. This is confusing so at a minimum one of them must be renamed. I am less attached to the actual name we use: "ExternalRepos" is a good description of the file, but if you have a better idea, let's use that.

Signed-off-by: Mike Thompson mike@openhwgroup.org