Clarify build instructions for the example

[secworks]

Trying to follow the instructions in the main README, and then the README in example just to get a default FPGA example bitstream does not work. The main README states:

$ cd gateware $ make

The main README build instructions results in a build directory with a muacm.v and muacm.ilang file. So far so good. But when following the instructions in example/README.md the build stops with the following error.

"make: *** No rule to make target 'acm-loopback.bin', needed by 'all'. Stop."

The build instructions in the README.md is:

make BOARD=bitsy-v1

It turns out that one instead need to run ./dist.sh in the repo root dir. This script creates a subdir under build/ with a name something like "muacm-example-20211027-f6fdeae". In that subdir one can then run make to actually build the example bitstream.

The dist.sh script will also perform the initial build performed by the build instructions in the main README.

I would like to suggest to update the build instructions to mention dist.sh and how to successfully build the default example design.

Also, for completeness sake, it might also help to recommend users to use "git submodule init" and "git submodule update" to populate the gateware subdirs with SERV etc.

[smunaut]

Mmm, yeah, the various READMEs (especially the one in example/) are written from the standpoint of someone using the distribution packages rather than starting from the raw sources from this repo.

I need to find a way to explain the difference between the two cases.

[secworks]

Cool! BTW I've just added a new consraint file for the Lattice UP Breakout Board, It works like a charm.