store user-visible image sizes in target-files

Do the yaffs-specific adjustments to image sizes in common.CheckSize,
instead of baking it into the image size stored in the target-files
package.  Remove the special fs_type flag and fold it into the
"info_dict" we have for saving key-value pairs from the build system.

Change-Id: I6e63f3330f6277d9a946b22e66cadeb51203ba14