Both STM32MP BootROM and TF-A first stage expect subsequent bootloader
stages to feature a specific STM32 file header. Generate this image
type by default.

If for some reason, the image without stm32 header is required, the
start_stm32mp157c_dk2.pblb can be used.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>