bugfix(freertos): CPU switch from kernel to user mode

components/freertos/port/esp8266/xtensa_vectors.S

@@ -1521,9 +1521,15 @@ _xt_ext_panic:
      *
      * Todo: Close NMI
      */
-    rsil    a0, XCHAL_EXCM_LEVEL
+    rsil    a0, PS_INTLEVEL(5)
     wsr     a0, EXCSAVE_2
 
+    /*
+     * CPU must switch from kernel mode to user mode
+     */
+    movi    a0, PS_INTLEVEL(5) | PS_UM
+    wsr     a0, PS
+
     /* Allocate exception frame and save minimal context. */
     mov     a0, sp
     movi    sp, _chip_interrupt_tmp