Add support to Print::printf for printing from flash (esp8266#2266)

* Add support to Print::printf for printing from flash

Author: me-no-dev

cores/esp8266/Print.cpp

@@ -63,6 +63,29 @@ size_t Print::printf(const char *format, ...) {
     return len;
 }
 
+size_t Print::printf_P(PGM_P format, ...) {
+    va_list arg;
+    va_start(arg, format);
+    char temp[64];
+    char* buffer = temp;
+    size_t len = vsnprintf_P(temp, sizeof(temp), format, arg);
+    va_end(arg);
+    if (len > sizeof(temp) - 1) {
+        buffer = new char[len + 1];
+        if (!buffer) {
+            return 0;
+        }
+        va_start(arg, format);
+        vsnprintf_P(buffer, len + 1, format, arg);
+        va_end(arg);
+    }
+    len = write((const uint8_t*) buffer, len);
+    if (buffer != temp) {
+        delete[] buffer;
+    }
+    return len;
+}
+
 size_t Print::print(const __FlashStringHelper *ifsh) {
     PGM_P p = reinterpret_cast<PGM_P>(ifsh);
 

cores/esp8266/Print.h

@@ -64,6 +64,7 @@ class Print {
         }
 
         size_t printf(const char * format, ...)  __attribute__ ((format (printf, 2, 3)));
+        size_t printf_P(PGM_P format, ...) __attribute__((format(printf, 2, 3)));
         size_t print(const __FlashStringHelper *);
         size_t print(const String &);
         size_t print(const char[]);