3ds: Various fixes, mostly for compiler warnings

- Move all devkitpro includes before the Rockbox ones so that the macros which are both conflicting and unused can be undef'd
- Remove unused result variables
- Exclude an unused function from being compiled for this target
- Fix hex number formatting
- Fix the return value of dummy functions
- Fix macro redefinition in the plugins keypad config
- Remove duplicate button mapping
- Turn off -Wchar-subscripts as it's already handled in Rockbox's ctype.h

Change-Id: I3f5a3d492c585f233277a380feaea5fe877a044f

firmware/kernel/thread-common.c

@@ -231,7 +231,7 @@ void format_thread_name(char *buf, size_t bufsize,
     snprintf(buf, bufsize, fmt, name, thread->id);
 }
 
-#ifndef HAVE_SDL_THREADS
+#if !defined(HAVE_SDL_THREADS) && !defined(CTRU)
 /*---------------------------------------------------------------------------
  * Returns the maximum percentage of the stack ever used during runtime.
  *---------------------------------------------------------------------------
@@ -252,7 +252,7 @@ static unsigned int stack_usage(uintptr_t *stackptr, size_t stack_size)
 
     return usage;
 }
-#endif /* HAVE_SDL_THREADS */
+#endif /* !defined(HAVE_SDL_THREADS) && !defined(CTRU) */
 
 #if NUM_CORES > 1
 int core_get_debug_info(unsigned int core, struct core_debug_info *infop)