Kernel: Make pseudo terminals not overwrite old data

If pseudo terminal buffer was filled, old implementation would overwrite
old data. This is bad if producer is capable of producing more data than
consumer can handle.

kernel/kernel/Terminal/Serial.cpp

@@ -258,9 +258,10 @@ namespace Kernel
 		return m_serial.height();
 	}
 
-	void SerialTTY::putchar_impl(uint8_t ch)
+	bool SerialTTY::putchar_impl(uint8_t ch)
 	{
 		m_serial.putchar(ch);
+		return true;
 	}
 
 }