Reserve sources.props for listing just the MicroPython core and extmod
files, similar to how py.mk lists port-independent source files. This
allows reusing the source list, for instance for building mpy-cross. The
sources for building the executable itself are listed in the corresponding
project file, similar to how the other ports specify the source files in
their Makefile.
Printing debugging info by defining MICROPY_DEBUG_VERBOSE expects
a definition of the DEBUG_printf function which is readily available
in printf.c so include that file in the build. Before this patch
one would have to manually provide such definition which is tedious.
For the msvc port disable MICROPY_USE_INTERNAL_PRINTF though: the
linker provides no (easy) way to replace printf with the custom
version as defined in printf.c.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
