minimal: Add enough code to run minimal build on STM32F4xx hardware.

Minimal support code for a Cortex-M CPU is added, along with set-up
code for an STM32F4xx MCU, including a UART for a REPL.  Tested on
a pyboard.  Code size is 77592 bytes.

1 files changed, 35 insertions, 0 deletions

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+# The minimal port
+
+This port is intended to be a minimal MicroPython port that actually runs.
+It can run under Linux (or similar) and on any STM32F4xx MCU (eg the pyboard).
+
+## Building and running Linux version
+
+By default the port will be built for the host machine:
+
+    $ make
+
+To run a small test script do:
+
+    $ make run
+
+## Building for an STM32 MCU
+
+The Makefile has the ability to build for a Cortex-M CPU, and by default
+includes some start-up code for an STM32F4xx MCU and also enables a UART
+for communication.  To build:
+
+    $ make CROSS=1
+
+If you previously built the Linux version, you will need to first run
+`make clean` to get rid of incompatible object files.
+
+Building will produce the build/firmware.dfu file which can be programmed
+to an MCU using:
+
+    $ make CROSS=1 deploy
+
+This version of the build will work out-of-the-box on a pyboard (and
+anything similar), and will give you a MicroPython REPL on UART1 at 9600
+baud.  Pin PA13 will also be driven high, and this turns on the red LED on
+the pyboard.