Revive target: ESP8266

This patch updates ESP8266 build system and code base as well.
 - Removed unnecessary files.
 - Decreased code size.
 - Refactored jerry_{port, extapi, run}.c to make it more easy to handle.
 - Readme.md is updated as well which contains detailed step by step description about how to set up environment and use JerryScript.

Finally, it solves the related issue #1375.

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

targets/esp8266/user/Makefile

@@ -37,7 +37,8 @@ endif
 # Required for each makefile to inherit from the parent
 #
 
-INCLUDES := $(INCLUDES) -I $(PDIR)include
-INCLUDES += -I ./ -I ../include -I../../../
-sinclude $(PDIR)Makefile
 
+INCLUDES := $(INCLUDES) -I $(PDIR)include
+INCLUDES += -I ./
+PDIR := ../$(PDIR)
+sinclude $(PDIR)Makefile