Fix File::readString to work with binary data (#8742)

Previously, File::readString used a C-style string as an intermediate
buffer via the String += operator. This treats a NUL byte as a
terminator, making this function work incorrectly if the File contains
binary data.

This commit switches the function to use String::concat, which doesn't
treat NUL bytes any differently (and is a bit faster, because it doesn't
need to use strlen).

cores/esp8266/FS.cpp

@@ -173,18 +173,15 @@ File File::openNextFile() {
     return _fakeDir->openFile("r");
 }
 
-String File::readString()
-{
+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);
-    }
+    uint8_t temp[256];
+    int countRead;
+    do {
+        countRead = read(temp, sizeof(temp));
+        ret.concat((const char*)temp, countRead);
+    } while (countRead > 0);
     return ret;
 }