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

apps/plugins/lib/keymaps.h

@@ -310,7 +310,8 @@
       (CONFIG_KEYPAD != ONDAVX777_PAD) && \
       (CONFIG_KEYPAD != CREATIVE_ZENXFI2_PAD) && \
       (CONFIG_KEYPAD != SHANLING_Q1_PAD) && \
-      (CONFIG_KEYPAD != HIBY_R3PROII_PAD)
+      (CONFIG_KEYPAD != HIBY_R3PROII_PAD) && \
+      (CONFIG_KEYPAD != CTRU_PAD)
 #define BTN_FIRE       BUTTON_BOTTOMLEFT
 #define BTN_PAUSE      BUTTON_TOPLEFT
 #endif

apps/plugins/plugin_crt0.c

@@ -28,7 +28,7 @@ PLUGIN_HEADER
 #ifdef CTRU
 /* dummy undefined symbols to build plugins for ctru */
 void __aeabi_unwind_cpp_pr0(void) {}
-struct _reent * _EXFUN(__getreent, (void)) {}
+struct _reent * _EXFUN(__getreent, (void)) { return NULL; }
 #endif
 
 /*