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

lib/rbcodec/codecs/codec_crt0.c

@@ -34,7 +34,7 @@ int _start(void) {return 0;}
 #ifdef CTRU
 /* dummy undefined symbols */
 void __aeabi_unwind_cpp_pr0(void) {}
-struct _reent * _EXFUN(__getreent, (void)) {}
+struct _reent * _EXFUN(__getreent, (void)) { return NULL; }
 #endif
 
 enum codec_status codec_start(enum codec_entry_call_reason reason)