zephyr/cmake: add new ${RIMAGE_TOP} constant

In preparation for changing it, see
   thesofproject#8178

No functional change yet.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

zephyr/CMakeLists.txt

@@ -58,11 +58,11 @@ set(SOF_IPC_PATH "${SOF_SRC_PATH}/ipc")
 set(SOF_DEBUG_PATH "${SOF_SRC_PATH}/debug")
 set(SOF_MATH_PATH "${SOF_SRC_PATH}/math")
 set(SOF_TRACE_PATH "${SOF_SRC_PATH}/trace")
+set(RIMAGE_TOP ${sof_top_dir}/rimage)
 
 # Save path to rimage configuration files in cmake cache for later use by
 # rimage during the "west sign" stage
-get_filename_component(RIMAGE_CONFIG "../rimage/config" ABSOLUTE)
-set(RIMAGE_CONFIG_PATH ${RIMAGE_CONFIG} CACHE PATH
+set(RIMAGE_CONFIG_PATH ${RIMAGE_TOP}/config} CACHE PATH
     " Path to rimage board configuration files")
 
 include(ExternalProject)
@@ -87,7 +87,7 @@ ExternalProject_Add(sof_logger_ep
 )
 
 # default SOF includes
-target_include_directories(SOF INTERFACE ${sof_top_dir}/rimage/src/include)
+target_include_directories(SOF INTERFACE ${RIMAGE_TOP}/src/include)
 target_include_directories(SOF INTERFACE ${SOF_SRC_PATH}/include)
 target_include_directories(SOF INTERFACE ${SOF_SRC_PATH}/arch/${ARCH}/include)
 target_include_directories(SOF INTERFACE ${sof_top_dir}/third_party/include)