fix build of menu-packages (do hardcode /usr)

d2cc121e0798a007a6e0409b09c16ce747ab959f broke building the menu
packages. The whole point of those is to have hardcoded location, so
that the system's desktop-environment will actually find the
menu/mimetype-definitions and icons

Change-Id: I940eb1243913d21d46d3579de30c8fabdc28fde3