stm32mp: stm32prog: add support of ssbl copy

For reliability of boot from NAND/SPI-NAND (with read-disturb issue)
the SSBL can be present several time, when it is indicated in the
flashlayout with "Binary(X)".
The received binary is copied X times by U-Boot on the target.

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