capsule: board: Add information needed for capsule updates

Add a structure which defines the information that is needed for
executing capsule updates on a platform. Some information in the
structure like the dfu string is used for making the update process
more robust while some information like the per platform image GUIDs
is used for fixing issues. Initialise this structure in the board
file, and use the information for the capsule updates.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>