board/pc/genimage-efi.cfg: align the rootfs to start at 16MB offset from start of the image

For grub-efi we can align the rootfs to start at exactly 16MB from the
beginning of the disk.  This can be done by reducing the vfat partition to
be 32 KB smaller than its 16MB size, just like the offset of the vfat
partition.

Signed-off-by: Florian La Roche <Florian.LaRoche@gmail.com>
[Peter: drop redundant offset/size settings, add comment]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>