platform-independent makefile: fix old "stm32" reference

the "make clean" target showed a constant "stm32/f1" debug message
instead of the current relative directory. this is fixed by showing the
relevant portion of the current directory instead, relevant being
determined by evaluating the current makefile's relative position and
stripping that part off.

lib/Makefile.include

@@ -42,7 +42,7 @@ $(SRCLIBDIR)/$(LIBNAME).ld: $(LIBNAME).ld
 	$(Q)$(CC) $(CFLAGS) -o $@ -c $<
 
 clean:
-	@printf "  CLEAN   lib/stm32/f1\n"
+	@printf "  CLEAN   lib$(subst $(shell cd $(dir $(lastword $(MAKEFILE_LIST))) && pwd),,$(shell pwd))\n"
 	$(Q)rm -f *.o *.d ../*.o ../*.d
 	$(Q)rm -f $(SRCLIBDIR)/$(LIBNAME).a
 	$(Q)rm -f $(SRCLIBDIR)/$(LIBNAME).ld