Improved readString() for File (#5445)

* added new boards

* corrected block size error

* added board to last commit working well

* override of readString for File class
Stream::readString requires a timeout of 1 seconds + multiple memory resize

* correct indent
return if size 0 before reserve

* correct indent

* good indent

* stricter test for end of string

* same implementation than Stream by replacing timeRead by read

* reading file by block of 256

* make sure there is an end of string

* fixed bug for file size multiple of 256
string::concate(char*) needs a string terminator to work

cores/esp8266/FS.cpp

@@ -121,6 +121,21 @@ const char* File::name() const {
     return _p->name();
 }
 
+String File::readString()
+{
+    String ret;
+    ret.reserve(size() - position());
+    char temp[256+1];
+    int countRead = readBytes(temp, sizeof(temp)-1);
+    while (countRead > 0)
+    {
+        temp[countRead] = 0;
+        ret += temp;
+        countRead = readBytes(temp, sizeof(temp)-1);
+    }
+    return ret;
+}
+
 File Dir::openFile(const char* mode) {
     if (!_impl) {
         return File();

cores/esp8266/FS.h

@@ -72,6 +72,7 @@ public:
     void close();
     operator bool() const;
     const char* name() const;
+    String readString() override;
 	
 protected:
     FileImplPtr _p;

cores/esp8266/Stream.h

@@ -101,7 +101,7 @@ class Stream: public Print {
         // returns the number of characters placed in the buffer (0 means no valid data found)
 
         // Arduino String functions to be added here
-        String readString();
+        virtual String readString();
         String readStringUntil(char terminator);
 
     protected: