Move sd init into diskio.c, write code for latest commit

fusee/fusee-secondary/src/sd_utils.h

@@ -4,7 +4,6 @@
 #include "utils.h"
 #include "sdmmc.h"
 
-int initialize_sd(void);
 size_t read_sd_file(void *dst, size_t dst_size, const char *filename);
 
 #endif