Do not download/build/clean libopencm3 directory when compiling for PC-hosted platforms.

Makefile

@@ -3,16 +3,28 @@ MFLAGS += --no-print-dir
 Q := @
 endif
 
+PC_HOSTED =
+ifeq ($(PROBE_HOST), libftdi)
+	PC_HOSTED = true
+endif
+ifeq ($(PROBE_HOST), pc-stlinkv2)
+	PC_HOSTED = true
+endif
+
 all:
+ifndef 	PC_HOSTED
 	$(Q)if [ ! -f libopencm3/Makefile ]; then \
 		echo "Initialising git submodules..." ;\
 		git submodule init ;\
 		git submodule update ;\
 	fi
 	$(Q)$(MAKE) $(MFLAGS) -C libopencm3 lib
+endif
 	$(Q)$(MAKE) $(MFLAGS) -C src
 
 clean:
+ifndef 	PC_HOSTED
 	$(Q)$(MAKE) $(MFLAGS) -C libopencm3 $@
+endif
 	$(Q)$(MAKE) $(MFLAGS) -C src $@