Update BUILDING.md

[kmatch98]

Add --recursive to update command.

[dhalbert]

We actually have recommended not to do --recursive, because it drags in a lot of stuff to TinyUSB and some other submodules that isn't needed. I do not do --recursive myself and my builds work, so I think the original problem discussed in discord may have been due to something else.

[kmatch98]

Ok, no problem. My builds were failing due to TinyUSB, so I had to make clean, then run the --recursive and then I could build properly. This was for the pyportal.

Feel free to ignore this if you think it was a different problem, but somehow this worked for me.

[dhalbert]

Discussed in discord # circuitpython, but for the record, this works on a fresh clone, without --recursive:

$ git clone https://github.com/adafruit/circuitpython
$ cd circuitpython/
$ git submodule update --init   # note no --recursive
$ cd ports/atmel-samd/
$ make BOARD=pyportal clean  # (unnecessary first time)
$ make BOARD=pyportal