The proper way is to change the contents of the PRODUCT_PACKAGES define to include su in one of the Android make files used to assemble the image for your device. You will need to change the LOCAL_MODULE_TAGS for the su project to optional. Be aware, as others have pointed out, your product will not pass CTS and qualify for Play Store licensing.
Here it is:
Under Operating System Software-Board Support Packages you can find an executable that will prepare your SD card under windows (SD_DEMO_IMAGE) or you can use the binary one (ANDROID_DEMO_IMAGE) under linux!