target/image: Use proper data types

target/image: Use proper data types

While at it, fix some coding style issues.

Change-Id: Id521394d89e0bf787a6f812701c2cc0fe7e4e63f
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: http://openocd.zylin.com/5919
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>