afboot-stm32: new package

This is a _very_ small bootloader for STM32 platforms.

At submission afboot-stm32 supports the following boards:

  stm32429i-eval
  stm32746g-eval
  stm32f429i-disco
  stm32f469i-disco

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Thomas: fix commit title, add dependency on BR2_arm.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>