The upcoming automatic update feature needs the ability to adjust an
address within Flash to the end of its respective sector. Factor out
this functionality to a new function flash_sect_roundb().

Signed-off-by: Rafal Czubak <rcz@semihalf.com>
Signed-off-by: Bartlomiej Sieka <tur@semihalf.com>
Signed-off-by: Stefan Roese <sr@denx.de>