fixes for compiling with gcc 10

gcc 10 no longer lets you get away with not externing global variables in header files. This adds the necessary extern and adds defines in appropriate c files

bootloader/storage/nx_sd.h

@@ -30,9 +30,9 @@ enum
 	SD_UHS_SDR82  = 3,
 };
 
-sdmmc_t sd_sdmmc;
-sdmmc_storage_t sd_storage;
-FATFS sd_fs;
+extern sdmmc_t sd_sdmmc;
+extern sdmmc_storage_t sd_storage;
+extern FATFS sd_fs;
 
 u32  sd_get_mode();
 int  sd_init_retry(bool power_cycle);