Merge branch 'mk-subs' into 'master'

Add submodules target to main makefile

See merge request redox-os/relibc!323

Makefile

@@ -51,7 +51,7 @@ WEAKEN_SYMBOLS=\
 	-W __umodti3 \
 	-W __rust_probestack
 
-.PHONY: all clean fmt install install-headers libs test
+.PHONY: all clean fmt install install-headers libs submodules test
 
 all: | libs
 
@@ -96,6 +96,10 @@ install-libs: libs
 
 install: install-headers install-libs
 
+submodules:
+	git submodule sync
+	git submodule update --init --recursive
+
 sysroot: all
 	rm -rf $@
 	rm -rf $@.partial