rp2/Makefile: Add "submodules" target, to match other ports.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/rp2/Makefile

@@ -20,3 +20,8 @@ all:
 
 clean:
 	$(RM) -rf $(BUILD)
+
+GIT_SUBMODULES += lib/pico-sdk lib/tinyusb
+
+submodules:
+	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$(GIT_SUBMODULES)" submodules

ports/rp2/README.md

@@ -29,6 +29,7 @@ Building of the RP2 firmware is done entirely using CMake, although a simple
 Makefile is also provided as a convenience.  To build the firmware run (from
 this directory):
 
+    $ make submodules
     $ make clean
     $ make