py/mkrules.mk: Fix 'make submodules' when building out-of-tree.

When MicroPython is used as a submodule and built from the containing
project, e.g. for the embed port, `make submodules` fails because it goes
looking for the sub-sub-module paths in the outer repository instead of in
the micropython repository. Fix this by invoking git inside the micropython
submodule.

Signed-off-by: Christian Walther <cwalther@gmx.ch>

py/mkrules.mk

@@ -251,8 +251,8 @@ endif
 submodules:
 	$(ECHO) "Updating submodules: $(GIT_SUBMODULES)"
 ifneq ($(GIT_SUBMODULES),)
-	$(Q)git submodule sync $(addprefix $(TOP)/,$(GIT_SUBMODULES))
-	$(Q)git submodule update --init $(addprefix $(TOP)/,$(GIT_SUBMODULES))
+	$(Q)cd $(TOP) && git submodule sync $(GIT_SUBMODULES)
+	$(Q)cd $(TOP) && git submodule update --init $(GIT_SUBMODULES)
 endif
 .PHONY: submodules