boot: zephyr: defines FLASH device for external NOR

With CONFIG_STM32_MEMMAP flag get NOR flash 32 bytes header
Read the NOR flash to get header of the image.
The FLASH_DEVICE is now the external NOR defined by
the board device tree to be the first qspi/ospi/xspi instance
(not necessarily zephyr, flash-controller)

Signed-off-by: Francois Ramu <francois.ramu@st.com>
Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>