To build:

make BOARD=ESPRUINO_PICO

To deploy: short the BOOT0/BTN contact on the back of the board (eg by
drawing over it with a graphite pencil), then hold down BTN while
inserting the board into the USB port.  The board should then enter DFU
mode, and the firmware can be downloaded using:

make BOARD=ESPRUINO_PICO deploy

Each board now needs an mpconfigboard.mk file which defines AF_FILE and
LD_FILE.

Also moved stm32f405.ld to boards/ directory to keep things organised.

Since this file is only valid for F405 MCUs, not generic F4xx.

It uses a 16k cache buffer and so the filesystem size is limited.

Much more portable this way.

This patch allows vstr_printf to use all the available space of a fixed
vstr buffer.  vstr_printf is a good alternative to snprintf.

Thanks to Henrik Sölver for this patch.

Saves 68 bytes.

Some files are excluded, otherwise a whole lot of files need converting.

So now all printing should go via either mp_plat_print or
mp_sys_stdout_print.

Previous to this patch the printing mechanism was a bit of a tangled
mess.  This patch attempts to consolidate printing into one interface.

All (non-debug) printing now uses the mp_print* family of functions,
mainly mp_printf.  All these functions take an mp_print_t structure as
their first argument, and this structure defines the printing backend
through the "print_strn" function of said structure.

Printing from the uPy core can reach the platform-defined print code via
two paths: either through mp_sys_stdout_obj (defined pert port) in
conjunction with mp_stream_write; or through the mp_plat_print structure
which uses the MP_PLAT_PRINT_STRN macro to define how string are printed
on the platform.  The former is only used when MICROPY_PY_IO is defined.

With this new scheme printing is generally more efficient (less layers
to go through, less arguments to pass), and, given an mp_print_t*
structure, one can call mp_print_str for efficiency instead of
mp_printf("%s", ...).  Code size is also reduced by around 200 bytes on
Thumb2 archs.