esp8266: Make default ESP_SDK variable work correctly.

When esp-open-sdk is built with STANDALONE=y (the default) then ESP_SDK
is set to the correct value, so that "make" just works.

esp8266/Makefile

@@ -8,7 +8,7 @@ include ../py/py.mk
 
 PORT = /dev/ttyACM0
 CROSS_COMPILE = xtensa-lx106-elf-
-ESP_SDK = $(shell $(CC) -print-sysroot)/..
+ESP_SDK = $(shell $(CC) -print-sysroot)/usr
 
 INC =  -I.
 INC += -I$(PY_SRC)