soc/xtensa/nxp_adsp/CMakeLists.txt: use new WEST_SIGN_OPTS variable

Align `soc/nxp_adsp` with new `WEST_SIGN_OPTS` option added to
`soc/intel_adsp` by recent commit d98a7c2 ("soc: xtensa: cmake: add
new WEST_SIGN_OPTS variable").

This allows per-board rimage customization at the CMake level. Example
in `zephyr/boards/xtensa/nxp_adsp_NEWBOARD/board.cmake`:

  set(WEST_SIGN_OPTS -- -c rimage/config/special.toml)

Signed-off-by: Marc Herbert <[email protected]>

Author: marc-hb

soc/xtensa/nxp_adsp/CMakeLists.txt

@@ -15,6 +15,6 @@ add_custom_target(zephyr.ri ALL
 add_custom_command(
   OUTPUT ${CMAKE_BINARY_DIR}/zephyr/zephyr.ri
   COMMENT "west sign --if-tool-available --tool rimage ..."
-  COMMAND  west sign --if-tool-available --tool rimage --build-dir ${CMAKE_BINARY_DIR}
+  COMMAND  west sign --if-tool-available --tool rimage --build-dir ${CMAKE_BINARY_DIR} ${WEST_SIGN_OPTS}
   DEPENDS ${CMAKE_BINARY_DIR}/zephyr/${KERNEL_ELF_NAME}
 )