With minor changes to adc.c and storage.c to support the F769.

USB serial is now working for F7.

Internal file storage is now working for F7.  The flash is laid out a bit
differently to the F4 - 4 x 32K, 1 x 128K with the rest 256K, so the
internal storage is 96K.

Added more pind definitions for STM32F7DISC board.  Made USART1 be the
default HWUART repl.  The STLINK usb connector also looks like a USB
serial port which is attached to USART1 on the STM32F7DISC.

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.

When loading the ELF binary to the board with a debugger, the debugger
needs to know at which point to start executing the code. Currently the
entry point defaults to the start of the .text section.
Signed-off-by: Sven Wegener <sven.wegener@stealer.net>

Stack is full descending and must be 8-byte aligned.  It must start off
pointing to just above the last byte of RAM.

Previously, stack started pointed to last byte of RAM (eg 0x2001ffff)
and so was not 8-byte aligned.  This caused a bug in combination with
alloca.

This patch also updates some debug printing code.

Addresses issue #872 (among many other undiscovered issues).

This basically shrinks the remaining size of flash in the portion
that goes after the internal flash drive.

This compiles and links, but hasn't been tested on a board
yet and even if it was run, it doesn't currently do anything.

Addresses issues #272 and #273.

* Issue #232

To partly address Issue #220.

This reverts commit a215b09c.

* Move gc_collect from main to py/gc.c
* Define GC's memory boundaries in linker script
* Issue #220