esp_hw_support: create esp_cpu

Create a esp_cpu header that contains CPU-related functions and
utilities.

components/lwip/port/esp32/hooks/tcp_isn_default.c

@@ -167,7 +167,7 @@ lwip_hook_tcp_isn(const ip_addr_t *local_ip, u16_t local_port,
    * APIs should not create any issues.
    */
 #if CONFIG_SPIRAM_ALLOW_STACK_EXTERNAL_MEMORY
-  assert(!esp_ptr_external_ram(get_sp()));
+  assert(!esp_ptr_external_ram(esp_cpu_get_sp()));
 #endif
 
   struct MD5Context ctx;