simplified message handling, switch from XOR checksum to CRC8 with polynomial 0x97.

0x97 allows for detection of all 1-3 bit errors in a message of up to
119 bytes
(see https://www.faa.gov/aircraft/air_cert/design_approvals/air_software/media/TC-14-49.pdf)

avr/com.asm

@@ -63,13 +63,15 @@ comDataBegin:
 
   comStatsRecvErrs:       .byte 2
   comStatsRecvCrcErrs:    .byte 2
-  comStatsCollisions:     .byte 2
   comStatsMissed:         .byte 2
-  comStatsAborted:        .byte 2
   comStatsIgnored:        .byte 2
   comStatsHandled:        .byte 2
+
+  comStatsCollisions:     .byte 2
+  comStatsAborted:        .byte 2
   
-  comStatsNoBuffer:       .byte 2
+  comStatsSendNoBuffer:   .byte 2
+  comStatsRecvNoBuffer:   .byte 2
 
   comRecvBuffersReadPos:  .byte 1
   comRecvBuffersWritePos: .byte 1
@@ -223,6 +225,8 @@ COM_AllocBufferAndGetXY:
   sec
   ret
 COM_AllocBufferAndGetXY_error:
+  ldi xl, LOW(comStatsSendNoBuffer)
+  ldi xh, HIGH(comStatsSendNoBuffer)
   clc
   ret