cmd_sf: let "sf update" preserve the final part of the last sector

cmd_sf: let "sf update" preserve the final part of the last sector

Since "sf update" erases the last block as a whole, but only rewrites
the meaningful initial part of it, the rest would be left erased,
potentially erasing meaningful information.
So, as a safety measure, have it rewrite the original content.

Signed-off-by: Gerlando Falauto <[email protected]>
Cc: Valentin Longchamp <[email protected]>
Cc: Holger Brunck <[email protected]>
Acked-by: Simon Glass <[email protected]>
Signed-off-by: Jagannadha Sutradharudu Teki <[email protected]>