Makefile and save() updated for NUCLEOF401RE

a-Makefile is now able to flash with the current user who connected the NucleoF401RE to the USB port
b-NUCLEOF401RE.py describes correctly the flash parameters: save() works correctly and flashing Espruino doesn't removes what has been saved.
c-seWatch doesn't work yet
d-Arduino's pins are not prefixed with 'Arduino.' yet
e-UART6 has to be checked