stm32mp: stm32prog: handle U-Boot script in flashlayout alternate

Update the stm32prog command to allow the reception of U-Boot script in
the FlashLayout alternate during the first USB enumeration.

This patch is aligned with the last TF-A behavior: the Flashlayout
is now loaded by U-Boot; it is no more present at STM32_DDR_BASE when
the stm32prog is launched after a serial boot, on UART or on USB.

The received script must be a U-Boot legacy image, no more need to add
a stm32image header.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>