unix/Makefile: Make install more compatible (BSD, etc.).

The current install command uses the flag -D which is specific to the
install command from GNU coreutils, but isn't available for the BSD
version. This solution uses the -d flag which should be commonly
available to create the target directory. Afterwards the target files
are installed to this directory seperately.

unix/Makefile

@@ -193,8 +193,9 @@ PIPSRC = ../tools/pip-micropython
 PIPTARGET = pip-micropython
 
 install: micropython
-	install -D $(TARGET) $(BINDIR)/$(TARGET)
-	install -D $(PIPSRC) $(BINDIR)/$(PIPTARGET)
+	install -d $(BINDIR)
+	install $(TARGET) $(BINDIR)/$(TARGET)
+	install $(PIPSRC) $(BINDIR)/$(PIPTARGET)
 
 # uninstall micropython
 uninstall: