Kconfig: change default toolchain prefix for esp32s2beta to "esp32s2"

Kconfig

@@ -43,7 +43,7 @@ mainmenu "Espressif IoT Development Framework Configuration"
         config SDK_TOOLPREFIX
             string "Compiler toolchain path/prefix"
             default "xtensa-esp32-elf-" if IDF_TARGET_ESP32
-            default "xtensa-lx7-elf-" if IDF_TARGET_ESP32S2BETA
+            default "xtensa-esp32s2-elf-" if IDF_TARGET_ESP32S2BETA
             help
                 The prefix/path that is used to call the toolchain. The default setting assumes
                 a crosstool-ng gcc setup that is in your PATH.