debug: create circular buffer for silent mode

Create a circular buffer that stored all debug messages accordingly.
If the debug buffer is full, reuse it. A method called flush_debug_buffer
is added to enable printing all messages to console if verbose mode is
configured.

The global variable debug_buffer_full is used to track if, when
flushing debug buffer to console, we need to flush both buffer sides.

If verbose boolean variable is set, all messages are printed to the
console after beeing stored in the buffer.

The size of the buffer is 50Kb, defined in debug.hh.

A function debugf that received a variable list of arguments
is defined so we can change some printf from boot sequence
to debugf call. A different name is used to prevent C overload.

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