wic: fix vfat partition sector count only if needed

wic: fix vfat partition sector count only if needed

VFAT rootfs partitions sector count would get updated always even if
there is no need. Since parition size in wic is expressed in MB, any
sub MB change will cause the generated partition image to be larger
than allocated space within the disk image. Fortunately, partitions
sized in MB will most of the time have a proper sector count.

(From OE-Core rev: 99bee4cb489800d74dea4d2158ff834413685f04)

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>