Added a couple of improvements to the Makefile and Readme:
* Restructured the Makefile to support multiple boards based on variable
* Verilator build is also done in Docker container with local option
* Restructured Readme to reflect changes in the Makefile
* Support for running the verilator chiselwatt binary in a Docker
container in case the OS is not Linux.
Signed-off-by: Carlos de Paula <me@carlosedp.com>