west: sign: do not make rimage configuration and platform name mandatory

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>