bootutil: Add cmake build file

In order to allow other projects to include the bootutil files more
easily. Allows renaming and moving of bootutil files without breaking
external projects' file lists (if they include this cmake file instead
of directly listing the files they use). Prevents an issue where
moving/renaming bootutil files breaks the FIH CI test.

Signed-off-by: Raef Coles <raef.coles@arm.com>
Change-Id: Ic982413c6a26ea2039712437f2d511fbe202e1e4