soc: stm32u5: Request header-size 1024 when using imgtool

On STM32U5 series, when an image is build with mcuboot,
image starts at offset 0x400 instead of default 0x200.

This should be taken into account when calling imgtool by using
dedicated option to set header-size value.


Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>