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/generic/misc.h

@@ -2,7 +2,6 @@
 #define __GENERIC_MISC_H
 
 #include <stdarg.h> // va_list
-#include <stddef.h> // size_t
 #include <stdint.h> // uint8_t
 
 struct command_encoder;