ARM: stm32: Add update_sf script to install U-Boot into SF on DHSOM

ARM: stm32: Add update_sf script to install U-Boot into SF on DHSOM

Add script to read U-Boot from SD card and write it to matching
locations in the SPI NOR, thus making the SPI NOR bootable. The
script erases the entire SPI NOR, including U-Boot environment,
to make sure the installation is clean. To retain environment
from current running U-Boot, run 'saveenv' after running the
'update_sf' script.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>