The definition of DEBUG_printf doesn't depend on
MICROPY_USE_INTERNAL_PRINTF so move it out of that preprocessor
block and compile it conditionally just depending on the
MICROPY_DEBUG_PRINTERS macro. This allows a port to use DEBUG_printf
while providing it's own printf definition.
This new config option allows to control whether MicroPython uses its own
internal printf or not (if not, an external one should be linked in).
Accompanying this new option is the inclusion of lib/utils/printf.c in the
core list of source files, so that ports no longer need to include it
themselves.
In other words, unix port now uses overriden printf(), instead of using
libc's. This should remove almost all dependency on libc stdio (which
is bloated).
