This changes the base element for build-image.sh to ubuntu-minimal, and
adds the simple-init element, to make this match the images used in the
gate.
It also adds an EXTRA_ELEMENTS environment variable that users can use
to add elements to the image. For example, local-config is useful to put
the current user's ssh keys in the image.
Last, it documents the things I needed to install to get it to run
correctly on Ubuntu; this is likely not an exhaustive list.
Change-Id: I2506c6722b77bb2f861d08e7a6da453dc2bbbfc2
26641b6561