esp32/Makefile: Force micropython-lib as a required submodule.

Also use mkrules.mk's submodule target rather than duplicating the call to
`submodule sync`.

Until we can find a way to use idf.py/cmake to discover submodules we have
no way to discover optional or board-specific submodules so need to err on
the side of including everything.

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

ports/esp32/Makefile

@@ -14,7 +14,11 @@ BAUD ?= 460800
 
 PYTHON ?= python3
 
-GIT_SUBMODULES += lib/berkeley-db-1.xx
+# Would be good to use cmake to discover submodules (see how rp2/Makefile does
+# it), but on ESP32 the same trick doesn't work because "idf.py build" fails
+# on berkeley-db dependency before printing out the submodule list.
+# For now just force the submodule dependencies here.
+GIT_SUBMODULES += lib/berkeley-db-1.xx lib/micropython-lib
 
 .PHONY: all clean deploy erase submodules FORCE
 
@@ -52,4 +56,4 @@ erase:
 	idf.py $(IDFPY_FLAGS) -p $(PORT) -b $(BAUD) erase_flash
 
 submodules:
-	git submodule update --init $(addprefix ../../,$(GIT_SUBMODULES))
+	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$(GIT_SUBMODULES)" submodules