wic/bootimg-efi: if fixed-size is set then use that for mkdosfs

This is a bit of a compatibility issue more than anything. Some devices
get upset if the FAT file system contains less blocks than the
partition.

The fixed-size argument is currently respected by the partition creation
step but not by the file system creation step. Let's make it so the file
system respects this value as well.

Signed-off-by: Randolph Sapp <rs@ti.com>