Make the dh_update_sd_to_sf script generic, rename it to dh_update_block_to_sf
and implement two specific dh_update_sd_to_sf and dh_update_emmc_to_sf scripts
which load U-Boot from either SD or eMMC and install it into SPI NOR.

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