feat(fwu): add basic definitions for GUID handling

The FWU metadata structure uses GUID's to identify the updatable
firmware images. Add some basic helper functions and
macros that would be used for working with the GUID datatype.

With the FWU feature enabled, these would then be used for image
identification and booting of images from a particular
bank(partition).

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: Ia54c0402d72b503d6abd1d94bc751cc14602cd39