How to build factory images:

# Do a normal build prep, then in the ANDROID_TOP_DIR run:
$ make -j$(nproc) dist
$ cd out/dist/
$ . ../../device/linaro/dragonboard/factory-images/generate-factory-images-package.sh

OR

# Install AOSP img package with an existing images. First download
# the image to the android build top directory and then run:
$ . ./device/linaro/dragonboard/factory-images/generate-factory-images-package.sh