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
Fix it to have other boards to keep compiling
Set Buffer size for FastSerial ports

When using Deluxe TX , TelemetrySerial != Serial and therfore the buffer
size should be specified
Fix to have RX to be still compiling