Both STM32MP BootROM and TF-A first stage expect subsequent bootloader
stages to feature a specific STM32 file header. Add a stm32image
utility to address this.

Signed-off-by: Marco Felsch <marco.felsch@gmail.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>