uml: Make deb-pkg build target build a Debian-style user-mode-linux package

From: Ryan Anderson <ryan@michonline.com>

Make the deb-pkg build target understand the "um" arch and set up the
package and directory structure to match a mainline-Debian style
user-mode-linux package.

This is primarily so that it stops matching, exactly, the naming
convention used by normal, non-UML kernels generated by this command.

Installing "linux-2.6.11" and "linux-2.6.11", where one is a UML kernel
doesn't do the right thing.  This fixes that.

Signed-off-by: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>