Uses attrtuple if it's enabled, otherwise just a normal tuple.

If you need the functionality of a namedtuple but will only make 1 or a
few instances, then use an attrtuple instead.

This fixes a long standing problem that viper code generation gave
terrible error messages, and actually no errors on pyboard where
assertions are disabled.

Now all compile-time errors are raised as proper Python exceptions, and
are of type ViperTypeError.

Addresses issue #940.

Addresses issue #1190.

The bootloader needs string0.c because of memset, memcpy and others,
without string0.c it magically links, but calling any of those
functions results in a hard fault.
In debug mode, modpyb needs printf, and including stdio.h leads to
conflicts due to the redefinitions made in the simplelink drivers.

Adds support for the following Thumb2 VFP instructions, via the option
MICROPY_EMIT_INLINE_THUMB_FLOAT:

vcmp
vsqrt
vneg
vcvt_f32_to_s32
vcvt_s32_to_f32
vmrs
vmov
vldr
vstr
vadd
vsub
vmul
vdiv

Addresses issue #1188.

Simply needed to define which pins have which ADC channel on them.

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.