install commands don't respect DESTDIR to install into a stage directory

[yurivict]

For example:

yosys-f4pga-plugins/xdc-plugin/Makefile:	install -D $< $(YOSYS_PLUGINS_DIR)/fasm_extra_modules/$<

should be

yosys-f4pga-plugins/xdc-plugin/Makefile:	install -D $< $(DESTDIR)$(YOSYS_PLUGINS_DIR)/fasm_extra_modules/$<

[timkpaine]

I can do this once I recover mentally from #485 😆