Add support for STM32F469 MCU in examples (closes #64)

[lionelains]

This PR allows running rustBoot and its example upgrade flow on the STM32F469-IDISCO board.

On this board, the sample rustBoot flash upgrade process is as follows: (it is executed when issuing cargo stm32f469 build-sign-flash rustBoot 1234 1235 from the command line)

• Once the demo is flashed on the STM32F469-IDISCO board, press the reset (black button) of the board to start the execution
• The green LED flashes 3 times (while the MCU is running the boot firmware corresponding to source file boards/firmware/stm32f469/boot_fw_blinky_green/src/main.rs)
• A delay of a few seconds occurs while the update partition is transferred to the boot partition by rustBoot
• The red LED flashes 3 times (the MCU is now running the update firmware corresponding to source file boards/firmware/stm32f469/updt_fw_blinky_red/src/main.rs)
• As soon as the new firmware sets the flash success state, the blue LED turns on while the red LED continues flashing forever

Note
When I mention color LEDs above, I mean the front LEDs (next to the LCD screen) of the STM32F469-IDISCO board.

[imrank03]

Acknowledged

[lionelains]

Hi @imrank03, yes, sure, I am going to propose an update to the doc as well...

Also, I think STM32 examples would benefit from factoring the flash lock/unlock/erase etc. part into a dedicated file.

[imrank03]

Hello @lionelains, sounds good! I am working on that.