Make Deluxe TX to work by removing FastSerial

Increase the default Serial RX buffer size
change mavlink.h as I did not know how to handle tx buffer size
Move serial_downlink at the bottom of the list to have openlrsng
configurator to display proper values (not shifted)
TODO :
-Need to change  openlrsng configurator to be able to deal with new
parameters
-Need to check the TX buffer of arduino serial
-need to run it properly on my deluxe tx that seems toasted because I
fired it up once without antenna ...
Use again fastserial but only for telem port

use the configurator from my fork as well