Isolating board dependancies

Hello,

I'm interesting on your project "Micropython". Using a different board than
those proposed, I focused on portability of your project on various STM boards.
(General portability on embbeded systems seems to be well done.)

So, I propose the following joined modifications. There is a little description of
these patches :
        * 1 : Linker script is board-dependant. This patch moves
        linker scripts on board/$(BOARD) directories, and adapts Makefile.
        * 2 : As the previous one, but about boot sequence. startup.s
        is moved on board/$(BOARD) directories, and Makefile is adapted.
        * 3 : We have to enable GPIO clocks one all the STM32 boards,
        but not the same clocks. main function will call a GPIO_CLK_ENABLE function,
        which is defined on board/$(BOARD) directories.
        * 4 : This patch may be a mistake. Are PYB_LED_R1...O defines
        usefull in led.h (from line 28) ? I don't think so. I propose to use less
        defines. This patch substitute all PYB_LED_? by PYB_LED_[1234] without
        regarding the board.
        * 5 : Work was well done for LCD, but not for UART. Configuration
        of UART is done in uart.c file. This patch moves configuration in
        uart_conf.h, in boards/$(BOARD) directories.

After this, I think your project is fully portable on every stm32 board, only by
adapting the board directory.

Hope this can help you.

Best regards,
Arnaud