cmd_sf: let "sf update" erase last sector as a whole

cmd_sf: let "sf update" erase last sector as a whole

make "sf update" work with unaligned `len' parameter, by deleting the
whole last sector before writing, so to allow for:

 sf update ${load_addr_r} 0 ${filesize}

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