New directory of scripts.

In particular, move some setup code for making sure HOL4 is running
from seL4-example/configure_default.sh into its own script.

Also copy in a debug script, which I've had privately for a long
while now. It's a collection of snippets which are useful to quote
to the interactive python interpreter to diagnose various failures.

Tweak auto-configure magic.

Discover some oddities. Probably fix some things.

Changed configure_default.sh to build an existing git checkout of poly
instead of checking it out itself

use $ISABELLE instead of assuming isabelle is in the users' PATH

Tweak summary of HOL4 build.

Tweak the way HOL4/polyml are built by default.

To get around a problem with recent HOL4 breaking at Holmake time
unless polyml contains a fix ( https://github.com/polyml/polyml/issues/54 )
we are tracking bleeding-edge polyml for the time being. Can change that
later.

Also tweaks to ways that HOL4's build is controlled and cleaning is
guesstimated.

Steps towards testing in l4v regression.

Handy setup script.

Automates default configuration of seL4-example, respecting existing
setup but setting up useful defaults.