BUILDING: lead with the submodules one board actually needs

The first thing a new contributor is told to run is fetch-all-submodules,
which pulls the submodules of every port. Measured on a checkout that has
them all, now that the broadcom port is gone: 5.7 GB of git data, against
501 MB for everything a raspberry_pi_pico build touches - about a
tenth of it.

fetch-port-submodules and fetch-board-submodules already exist and do the
narrow thing, and CI uses the same resolver. They were just the second
option in the text, and the board one was not mentioned at all; `make help`
listed only the fetch-all target.

So this reorders the section to lead with the board target, keeps the port
one for people building several boards of a port, and leaves fetch-all for
people who really do build more than one port. No behaviour change.

Author: lynt-smitka

BUILDING.md

@@ -25,13 +25,20 @@ Please ensure you set up your build environment appropriately, as per the guide.
 
 This project has a bunch of git submodules.  You will need to update them regularly.
 
-In the root folder of the CircuitPython repository, execute the following:
+Fetch only what the board you are building needs. From the root folder of the CircuitPython
+repository:
 
-    make fetch-all-submodules
+    make -C ports/raspberrypi fetch-board-submodules BOARD=raspberry_pi_pico
+
+Or everything one port needs, which is what you want if you build several boards from it:
 
-Or, in the ports directory for the particular port you are building, do:
+    make -C ports/raspberrypi fetch-port-submodules
 
-    make fetch-port-submodules
+There is also a target that fetches the submodules of every port at once. That is a few gigabytes
+against a few hundred megabytes for one board, so reach for it only if you really are building for
+more than one port:
+
+    make fetch-all-submodules
 
 ### Required Python Packages
 

Makefile

@@ -93,8 +93,13 @@ help:
 	@echo "  pseudoxml  to make pseudoxml-XML files for display purposes"
 	@echo "  linkcheck  to check all external links for integrity"
 	@echo "  doctest    to run all doctests embedded in the documentation (if enabled)"
-	@echo "  fetch-all-submodules	to fetch submodules for all ports"
+	@echo "  fetch-all-submodules	to fetch submodules for all ports (several GB)"
 	@echo "  remove-all-submodules	remove all submodules, including files and .git/ data"
+	@echo ""
+	@echo "To build one board you only need its own submodules, which is a much smaller"
+	@echo "download. From a port directory:"
+	@echo "  make fetch-board-submodules BOARD=<board>	submodules for one board"
+	@echo "  make fetch-port-submodules			submodules for every board of that port"
 
 clean:
 	rm -rf $(BUILDDIR)/*