Separate runtime debug output from debug symbols

With this change debug and debug_symbols is used consistently.
With 'make debug=yn debug_symbols=yn' my rebuild time and diskspace
usage goes down:
   105275K clean xen.git checkout.
y 1247041K build time: 5:57
n  812112K build time: 5:34

No change in behaviour is expected by that patch for tools and xen.
For stubdom CFLAGS change from -O1 to O2 with debug=n.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: David Scott <dave.scott@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Tim Deegan <tim@xen.org>
Cc: Wei Liu <wei.liu2@citrix.com>
autogen.sh

Signed-off-by: Olaf Hering <olaf@aepfle.de>