Building debug kernels (with additional run time checks and debugging)
was broken a few commits back. This adds back support for building debug
kernels.
In addition, it builds and uploads debug kernels for selected kernel
series (4.9.x LTS and latest stable). The tag for these kernels has
a "_dbg" suffix.
Update documentation.
Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>