cmake/romfs: separate c flags to avoid compile fail

riscv-none-elf-gcc: warning:  -Wstrict-prototypes: linker input file unused because linking not done
riscv-none-elf-gcc: error:  -Wstrict-prototypes: linker input file not found: No such file or directory

Signed-off-by: chao an <anchao@lixiang.com>

cmake/nuttx_add_romfs.cmake

@@ -59,11 +59,12 @@ function(nuttx_add_romfs)
 
   foreach(rcsrc ${RCSRCS})
     get_filename_component(rcpath ${rcsrc} DIRECTORY)
+    separate_arguments(CMAKE_C_FLAG_ARGS NATIVE_COMMAND ${CMAKE_C_FLAGS})
     add_custom_command(
       OUTPUT ${rcsrc}
       COMMAND ${CMAKE_COMMAND} -E make_directory ${rcpath}
       COMMAND
-        ${CMAKE_C_COMPILER} ${CMAKE_C_FLAGS} -E -P -x c
+        ${CMAKE_C_COMPILER} ${CMAKE_C_FLAG_ARGS} -E -P -x c
         -I${CMAKE_BINARY_DIR}/include ${CMAKE_CURRENT_SOURCE_DIR}/${rcsrc} >
         ${rcsrc}
       DEPENDS nuttx_context ${CMAKE_CURRENT_SOURCE_DIR}/${rcsrc})