Now, if the rimage target (meaning rimage configuration and platform name)
is not defined in board.cmake the sign script returns fatal error.
Change this to a warning since there are configurations that are
not using 'west sign' or is used just to glue the headers of the
final image.
Also, update the documentation accordingly.
Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Signed-off-by: Marc Herbert <marc.herbert@intel.com>