add macro for vsnprintf_P and snprintf_P

Author: Links2004

hardware/esp8266com/esp8266/cores/esp8266/pgmspace.h

@@ -7,6 +7,9 @@
 #define PGM_P  		const char *
 #define PSTR(str) 	(str)
 
+#define vsnprintf_P(...) 	vsnprintf( __VA_ARGS__ )
+#define snprintf_P(...) 	snprintf( __VA_ARGS__ )
+
 #define _SFR_BYTE(n) (n)
 
 typedef void 			prog_void;