toaster: improve image file suffix retrieval

toaster: improve image file suffix retrieval

Refactor retrieval of suffix from image file path, making it a
a method on Target_Image_File. This makes it easier to use this
in the build dashboard for individual images, plus reduces the
complexity of the code required to get all of the image file
suffixes for a build.

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: bavery <brian.avery@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>