[cmake] respect OT_POSIX_SETTINGS_PATH option (#842)

diff --git a/third_party/openthread/CMakeLists.txt b/third_party/openthread/CMakeLists.txt
index fe23713..4623fa5 100644
--- a/third_party/openthread/CMakeLists.txt
+++ b/third_party/openthread/CMakeLists.txt
@@ -79,7 +79,9 @@
     set(OT_DNSSD_SERVER ON CACHE BOOL "enable DNS-SD server support" FORCE)
 endif()
 
-list(APPEND OT_PLATFORM_DEFINES "-DOPENTHREAD_CONFIG_POSIX_SETTINGS_PATH=\"/var/lib/thread\"")
+if (NOT OT_POSIX_SETTINGS_PATH)
+    set(OT_POSIX_SETTINGS_PATH "\"/var/lib/thread\"" CACHE STRING "set the directory to store Thread data" FORCE)
+endif()
 
 add_subdirectory(repo EXCLUDE_FROM_ALL)