command: Prefer uint8_t* for buffers; prefer uint8_fast_t for lengths

Prefer using 'uint8_t' buffers as it is too easy to run into C sign
extension problems with 'char' buffers.  Prefer using 'uint_fast8_t'
for buffer lengths as gcc does a better job compiling them on 32bit
mcus.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/main.c

@@ -54,7 +54,7 @@ DECL_INIT(prescaler_init);
 
 // Optimized crc16_ccitt for the avr processor
 uint16_t
-crc16_ccitt(char *buf, uint8_t len)
+crc16_ccitt(uint8_t *buf, uint_fast8_t len)
 {
     uint16_t crc = 0xFFFF;
     while (len--)