debug: Change calls to printf on boot messages

Change some printf calls on boot messages, so it will call
the apropriate debug function. This will enable OSv
to operate on silent mode.

Added debug.h header so we can link debug functions to C files.

Fixes #118

Signed-off-by: Eduardo Piva <efpiva@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>