py/mkrules.mk: Use partial clone for submodules if available.

MicroPython relies on a number of submodules for third party and chip
vendor libraries.  Users need to check these out before building their
desired ports and Github Actions CI here needs to clone them all multiple
times for every build.  Many of these are getting significantly larger over
time, slowing down usage and consuming more disk space.

Newer versions of git have features to avoid pulling all historic / blob
data which can have a significant impact of total data use.  This commit
uses a standard feature of git to do a partial clone, with automatic
fallback to previous behavior on error.

Signed-off-by: Andrew Leech <[email protected]>

py/mkrules.mk

@@ -252,7 +252,11 @@ submodules:
 	$(ECHO) "Updating submodules: $(GIT_SUBMODULES)"
 ifneq ($(GIT_SUBMODULES),)
 	$(Q)cd $(TOP) && git submodule sync $(GIT_SUBMODULES)
-	$(Q)cd $(TOP) && git submodule update --init $(GIT_SUBMODULES)
+	# If available, do blobless partial clones of submodules to save time and space.
+	# A blobless partial clone lazily fetches data as needed, but has all the metadata available (tags, etc.).
+	# Fallback to standard submodule update if blobless isn't available (earlier than 2.36.0)
+	$(Q)cd $(TOP) && git submodule update --init --filter=blob:none $(GIT_SUBMODULES) || \
+	  git submodule update --init $(GIT_SUBMODULES)
 endif
 .PHONY: submodules