3ds: Port refactor.

This commit does the following changes to the 3ds port:

- Rename the target from ctru to 3ds.
- Rename all files and functions with the ctru naming convention to 3ds.
- Created a new file and folder structure that will better integrate future console ports that share the same codebase.
- Fixed a buffer overflow bug in pcm code.

Change-Id: I17c6f86df64eb99dd2b653485d70832ff46b2ba8

lib/rbcodec/codecs/codec_crt0.c

@@ -30,7 +30,7 @@ extern unsigned char plugin_end_addr[];
  * avoid warning with certain compilers */
 int _start(void) {return 0;}
 
-#ifdef CTRU
+#ifdef N3DS
 /* dummy undefined symbols */
 void __aeabi_unwind_cpp_pr0(void) {}
 struct _reent * _EXFUN(__getreent, (void)) { return NULL; }