multiboot: Refactor duplicated functions to a separate file

The implementation of write_bootdata() and get_redirect_dir() was
copied verbatim in two different places, obviously a bad thing for
maintainability. This moves them to a new file multiboot.c as they
are only used for multiboot.

Change-Id: Id0279216e4dd019f8bf612a81d3835eff010e506

firmware/export/mi4-loader.h

@@ -21,6 +21,9 @@
  *
  ****************************************************************************/
 
+#ifndef __MI4_LOADER_H__
+#define __MI4_LOADER_H__
+
 #include <stdint.h>
 
 #define MI4_HEADER_SIZE     0x200
@@ -50,21 +53,4 @@ struct tea_key {
 int load_mi4(unsigned char* buf, const char* firmware, unsigned int buffer_size);
 const char *mi4_strerror(int8_t errno);
 
-#ifdef HAVE_MULTIBOOT /* defined by config.h */
-/* Check in root of this <volume> for rockbox_main.<playername>
- * if this file empty or there is a single slash '/'
- * buf = '<volume#>/<rootdir>/<firmware(name)>\0'
- * If instead '/<*DIRECTORY*>' is supplied
- * addpath will be set to this DIRECTORY buf =
- * '/<volume#>/addpath/<rootdir>/<firmware(name)>\0'
- * On error returns Negative number or 0
- * On success returns bytes from snprintf
- * and generated path will be placed in buf
- * note: if supplied buffer is too small return will be
- * the number of bytes that would have been written
- */
-
-/* TODO needs mapped back to debug_menu if root redirect ever becomes a reality */
-int get_redirect_dir(char* buf, int buffer_size, int volume,
-                     const char* rootdir, const char* firmware);
-#endif
+#endif /* __MI4_LOADER_H__ */