build: convert debug to Kconfig

build: convert debug to Kconfig

Enabling debug will disable NDEBUG which will result in more debug
prints.  There are a number of debugging options for Xen so place the
debug option under a menu for different debugging options to have a way
to group them all together.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>