Use the SKD included in esp-open-sdk, if available

Makefile

@@ -55,8 +55,15 @@ ESP_HOSTNAME        ?= esp-link
 XTENSA_TOOLS_ROOT ?= $(abspath ../esp-open-sdk/xtensa-lx106-elf/bin)/
 
 # Base directory of the ESP8266 SDK package, absolute
-# Typically you'll download from Espressif's BBS, http://bbs.espressif.com/viewforum.php?f=5
+ifneq ($(wildcard ../esp-open-sdk/esp_iot_sdk_v1.5.2/.*),)
+# One that has been extracted as part of open-sdk
+SDK_BASE	?= $(abspath ../esp-open-sdk/esp_iot_sdk_v1.5.2)
+else
+ifneq ($(wildcard ../esp_iot_sdk_v1.5.1/.*),)
+# Manually downloaded from Espressif's BBS, http://bbs.espressif.com/viewforum.php?f=5
 SDK_BASE	?= $(abspath ../esp_iot_sdk_v1.5.1)
+endif
+endif
 
 # Esptool.py path and port, only used for 1-time serial flashing
 # Typically you'll use https://github.com/themadinventor/esptool