esp32/mpconfigport: Use the appropriate wait-for-interrupt opcode.

When threading is disabled, the pending events handling code would wait
for an incoming interrupt once there's no more work to do.  This bit of
code was Xtensa-specific and wouldn't compile on a RISC-V based MCU.

This commit provides the RISC-V equivalent to that part of the code,
allowing to make threadless MicroPython builds on RISC-V based MCUs.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

ports/esp32/mpconfigport.h

@@ -292,12 +292,17 @@ void *esp_native_code_commit(void *, size_t, void *);
         MP_THREAD_GIL_ENTER(); \
     } while (0);
 #else
+#if CONFIG_IDF_TARGET_ARCH_RISCV
+#define MICROPY_PY_WAIT_FOR_INTERRUPT asm volatile ("wfi\n")
+#else
+#define MICROPY_PY_WAIT_FOR_INTERRUPT asm volatile ("waiti 0\n")
+#endif
 #define MICROPY_EVENT_POLL_HOOK \
     do { \
         extern void mp_handle_pending(bool); \
         mp_handle_pending(true); \
         MICROPY_PY_SOCKET_EVENTS_HANDLER \
-        asm ("waiti 0"); \
+            MICROPY_PY_WAIT_FOR_INTERRUPT; \
     } while (0);
 #endif