[ax25-layer2] extended ax25

[Andre PE1RDW]

As long as we are re-evaluating the level 2 I think it would be smart to
elevate extended ax25 framing to default ax25 framing (larger frames and
frame numbering)
This helps a lot in high speed links to reduce the relative level 1
overhead like txd etc.
Lets face it, at 38K4 and higher 7 frames of 255 bytes are just to short.

73 de Andre PE1RDW