deb configure: cleanup merge of master

Support for Debian-7.0 was merged with Debian-6.0 in master, no need to
add it independently in this branch.
deb configure: flatten a nested if for legibility
deb configure: comment the inspect_kernel function
deb configure: re-enable "-a"

* Look at the running kernel, then at all the kernels with configs
    in /boot.

* Inspect each one for realtime extensions, use the first one we find
    that has any.

* Fail if we don't find one.
deb configure: most settings are per kernel-type, not per kernel

"kernel-type" ($KTYPE) is "rtai", "xenomai", "rt-preempt", or "vanilla".
Vanilla is non-realtime, what we used to call "sim".
deb configure: better success report from configure

There are three factors that affect how debian/configure configures the
debian package:

* distribution (eg Lucid or Precise)

* kernel type (eg vanilla or rtai or xenomai)

* kernel version

So report these things clearly.

Also: get rid of an unused substitution variable, it's just clutter.