command: Add command_decode_ptr() helper

Add a helper function to convert from a string buffer passed in the
args[] parameter to an actual pointer.  This avoids all the callers
needing to perfrom pointer manipulation.

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

src/command.h

@@ -71,6 +71,7 @@ enum {
 };
 
 // command.c
+void *command_decode_ptr(uint32_t v);
 uint8_t *command_parsef(uint8_t *p, uint8_t *maxend
                         , const struct command_parser *cp, uint32_t *args);
 uint_fast8_t command_encodef(uint8_t *buf, const struct command_encoder *ce