add toolchain setting for esp32s2

Kconfig

@@ -40,7 +40,8 @@ mainmenu "Espressif IoT Development Framework Configuration"
     menu "SDK tool configuration"
         config SDK_TOOLPREFIX
             string "Compiler toolchain path/prefix"
-            default "xtensa-esp32-elf-"
+            default "xtensa-esp32-elf-" if IDF_TARGET_ESP32
+            default "xtensa-lx7-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.

tools/cmake/toolchain-esp32s2beta.cmake

@@ -0,0 +1,9 @@
+set(CMAKE_SYSTEM_NAME Generic)
+
+set(CMAKE_C_COMPILER xtensa-lx7-elf-gcc)
+set(CMAKE_CXX_COMPILER xtensa-lx7-elf-g++)
+set(CMAKE_ASM_COMPILER xtensa-lx7-elf-gcc)
+
+set(CMAKE_EXE_LINKER_FLAGS "-nostdlib" CACHE STRING "Linker Base Flags")
+set(CMAKE_C_FLAGS "-mlongcalls" CACHE STRING "C Compiler Base Flags")
+set(CMAKE_CXX_FLAGS "-mlongcalls" CACHE STRING "C++ Compiler Base Flags")