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/generic/crc16_ccitt.c

@@ -8,7 +8,7 @@
 
 // Implement the standard crc "ccitt" algorithm on the given buffer
 uint16_t
-crc16_ccitt(char *buf, uint8_t len)
+crc16_ccitt(uint8_t *buf, uint_fast8_t len)
 {
     uint16_t crc = 0xffff;
     while (len--) {