It seems more and more likely that your post about the USB Serial not being compiled along with the main code is spot on.
I uploaded the binary of the above sketch (compiled using arduino IDE) using maple_upload and was able to observe the serial output in terminal monitor.
Even platformIO was able to see the serial port!!.
Next I compiled the binary using the platformio and uploaded from within the platformio IDE (using the same maple_upload and dfu upload).
But this time the board was stuck and the serial port was lost. The board as reported earlier, reverted to the six quick flashes and slow flashing and then stopped altogether.
Unfortunately platformio uses Scons for the build engine, which is beyond my immediate understanding to manipulate.
Hence I requested a complete command of compiling the binary. I wanted to compile the same binary manually using the toolchain packaged along with platformio and check the results.. then see what is the difference in the Scons script and the STM32duino compilation process.
Thank you for the help so far roger!!