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/pru/internal.h

@@ -25,7 +25,7 @@
 // Layout of shared memory
 struct shared_response_buffer {
     uint32_t count;
-    char data[MESSAGE_MAX];
+    uint8_t data[MESSAGE_MAX];
 };
 struct shared_mem {
     uint32_t signal;
@@ -36,7 +36,7 @@ struct shared_mem {
     const struct command_parser *command_index;
     uint32_t command_index_size;
     const struct command_parser *shutdown_handler;
-    char read_data[512];
+    uint8_t read_data[512];
 };
 
 #define SIGNAL_PRU0_WAITING 0xefefefef