stm32mp: stm32prog: handle flashlayout without STM32 image header

Accept flashlayout without header in alternate 0, to simplify
the support of stm32prog command with dfu-util.

By default the flashlayout file size is the size of the received binary,
provided with the offset in the DFU alternate.

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>