fix(build): Another check w/ upstrream shields.

* Properly handle not finding a shield in the board roots.

app/cmake/ZephyrBuildConfig.cmake

@@ -114,8 +114,9 @@ endforeach()
 # overlay/conf/keymap files.
 if(DEFINED SHIELD)
 	foreach(s ${SHIELD_AS_LIST})
-		message(STATUS ${SHIELD_DIR_${s}})
+		if (DEFINED $SHIELD_DIR_${s})
 			get_filename_component(shield_dir_name ${SHIELD_DIR_${s}} NAME)
+		endif()
 		string(REPLACE "_" ";" S_PIECES ${s})
 		list(LENGTH S_PIECES S_PIECES_LEN)
 		while(NOT S_PIECES STREQUAL "")