Gen the image dependency in the Graphviz dot format

By using

    ./tools/build.py --save-dependency a.dot

to gen the Graphviz dot dependency file. Later, you can use the `dot`
to draw the picture.

    dot -Tjpg -o a.jpg a.dot

Implements: blueprint images-dependency-tree
Depends-On: I8e07a1b69fab5f1c587470bfd2104aaba93f0050
Change-Id: If00f4f3fb9d0b10a07ab2abb7ffb1cd9d64902f2