flash: factor out adjusting of Flash address to the end of sector

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>